RECOMMENDED PRACTICE
ISA–RP12.24.01–1998 (IEC 79-10 Mod)
Recommended Practice for
Classification of Locations for
Electrical Installations
Classified as Class I, Zone 0,
Zone 1, or Zone 2
Approved 30 November 1997
ISA-RP12.24.01 (IEC 79-10 Mod), Recommended Practice for Classification of Locations for
Electrical Installations Classified as Class I, Zone 0, Zone 1, or Zone 2
ISBN: 1-55617-649-X
Copyright  1998 by the Instrument Society of America. All rights reserved. Printed in the
United States of America. No part of this publication may be reproduced, stored in a retrieval
system, or transmitted, in any form or by any means (electronic, mechanical, photocopying,
recording, or otherwise), without the prior written permission of the Publisher.
ISA
67 Alexander Drive
P. O. Box 12277
Research Triangle Park, North Carolina 27709
Preface
This ISA recommended practice is based on IEC Publication 79-10. It is the intention of the ISA
SP12 Committee to develop an ISA Recommended Practice that is harmonized with
IEC 79-10 to the fullest extent possible.
This preface, as well as all footnotes and annexes, is included for informational purposes and is
not part of ISA-RP12.24.01 (IEC 79-15 Mod).
This recommended practice has been prepared as part of the service of ISA, the international
society for measurement and control, toward a goal of uniformity in the field of instrumentation.
To be of real value, this document should not be static, but should be subject to periodic review.
Toward this end, the Society welcomes all comments and criticisms, and asks that they be
addressed to the Secretary, Standards and Practices Board; ISA; 67 Alexander Drive; P. O. Box
12277; Research Triangle Park, NC 27709; Telephone (919) 549-8411; Fax (919) 549-8288;
E-mail: standards.info@isa.org.
The ISA Standards and Practices Department is aware of the growing need for attention to the
metric system of units in general, and the International System of Units (SI) in particular, in the
preparation of instrumentation standards, recommended practices, and technical reports. The
Department is further aware of the benefits to U.S. users of ISA standards of incorporating
suitable references to the SI (and the metric system) in their business and professional dealings
with other countries. Toward this end, this Department will endeavor to introduce SI-acceptable
metric units in all new and revised standards to the greatest extent possible. Standard for Use of
the International System of Units (SI): The Modern Metric System, published by the American
Society for Testing & Materials as IEEE/ASTM SI 10-97, and future revisions, will be the
reference guide for definitions, symbols, abbreviations, and conversion factors.
It is the policy of ISA to encourage and welcome the participation of all concerned individuals and
interests in the development of ISA standards. Participation in the ISA standards-making
process by an individual in no way constitutes endorsement by the employer of that individual, of
the ISA, or of any of the standards, recommended practices, and technical reports that ISA
develops.
CAUTION—THE USE OF THIS STANDARD, RECOMMENDED PRACTICE, OR
TECHNICAL REPORT MAY INVOLVE HAZARDOUS MATERIALS, OPERATIONS OR
EQUIPMENT. THE STANDARD, RECOMMENDED PRACTICE, OR TECHNICAL
REPORT CANNOT ANTICIPATE ALL POSSIBLE APPLICATIONS OR ADDRESS
ALL POSSIBLE SAFETY ISSUES ASSOCIATED WITH USE IN HAZARDOUS
CONDITIONS.
THE USER OF THIS STANDARD, RECOMMENDED PRACTICE, OR TECHNICAL
REPORT MUST EXERCISE SOUND PROFESSIONAL JUDGMENT CONCERNING
ITS USE AND APPLICABILITY UNDER THE USER’S PARTICULAR
CIRCUMSTANCES. THE USER MUST ALSO CONSIDER THE APPLICABILITY OF
ANY GOVERNMENTAL REGULATORY LIMITATIONS AND ESTABLISHED SAFETY
AND HEALTH PRACTICES BEFORE IMPLEMENTING THIS STANDARD,
RECOMMENDED PRACTICE, OR TECHNICAL REPORT.
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
3
ADDITIONALLY, IMPLEMENTATION OF THE STANDARD, RECOMMENDED
PRACTICE, OR TECHNICAL REPORT MAY REQUIRE USE OF TECHNIQUES,
PROCESSES, OR MATERIALS COVERED BY PATENT RIGHTS. ISA TAKES NO
POSITION ON THE EXISTENCE OR VALIDITY OF ANY PATENT RIGHTS WHICH
MAY BE INVOLVED IN IMPLEMENTING THE STANDARD, RECOMMENDED
PRACTICE, OR TECHNICAL REPORT. ISA WILL NOT BE RESPONSIBLE FOR
IDENTIFYING ALL PATENTS THAT MAY REQUIRE A LICENSE BEFORE
IMPLEMENTATION OF THE STANDARD, RECOMMENDED PRACTICE, OR
TECHNICAL REPORT OR FOR INVESTIGATING THE VALIDITY OR SCOPE OF
ANY PATENTS BROUGHT TO ITS ATTENTION. THE USER SHOULD CAREFULLY
INVESTIGATE RELEVANT PATENTS BEFORE USING THE STANDARD,
RECOMMENDED PRACTICE, OR TECHNICAL REPORT FOR THE USER’S
INTENDED APPLICATION.
The following members of ISA SP12.24 contributed to the development of this document:
NAME
COMPANY
A. Ballard, Chairman
N. Abbatiello
D. Ankele
S. Arnold
D. Bishop
P. Dobler
W. Fiske
D. Jagger
B. Larson
W. Lawrence
R. Masek
D. Mohla
J. Propst
C. Sawyer
D. Wechsler
Crouse-Hinds Division of Cooper Ind.
Eastman Kodak Co.
Underwriters Laboratories Inc.
Drexelbrook Engineering Co.
Chevron USA Production Company
Weidmuller Inc.
Intertek Testing Services
HAWKE America
Turck Inc.
Factory Mutual Research Corp.
Bailey Controls Inc.
Union Carbide Chem. and Plastics
Shell Oil Products Company
MI Cable Co., Inc.
Union Carbide Corporation
The following people served as members of ISA Committee SP12:
NAME
COMPANY
*F. McGowan, Chairman
*D. Bishop, Managing Director
*N. Abbatiello
B. Apel
S. Arnold
*P. Babiarz
*A. Ballard
G. Bentinck
*R. Berthold
H. Bockle
K. Boegli
J. Bossert
R. Brodin
M. Buettner
R. Buschart
Factory Mutual Research Corp.
Chevron USA Production Company
Eastman Kodak Company
MSA Instrument
Drexelbrook Engineering Company
Crouse-Hinds Company
Crouse-Hinds Company
Dupont Engineering
Compression Systems Inc.
Killark/Stahl Inc.
Phoenix Contact Inc.
Hazloc, Inc.
Fisher Controls International, Inc.
Ralston Purina Company
PC & E, Inc.
4
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
*W. Calder III
R. Cardinal
C. Casso
R. Castillo
J. Cawley
H. Conner
M. Coppler
J. Cospolich
J. Costello
*E. Cranch
A. Czyz
W. Dill
P. Dobler
T. Dubaniewicz Jr.
U. Dugar
R. Ellis
A. Engler
T. Feindel
W. Fiske
S. Florence
G. Garcha
E. Geissler
B. Gibson
J. Greenwald
L. Hamman
E. Henning
D. Hohenstein
C. Hoy
*D. Jagger
X. Jianping
D. Kaplan
*P. Kelly
F. Kent
G. Kozinski
J. Kuczka
B. Larson
*W. Lawrence
W. Leber
T. Lewis Jr.
*N. Ludlam
V. Maggioli
*E. Magison
*F. Maltby
R. Masek
D. McDermott
I. McMurchie
*R. McNeal
J. Miller
*A. Mobley
M. Morrow
W. Mueller
E. Nesvig
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
Calder Enterprises.
Bently Nevada Corp.
Schlumberger Oil Field Services
Industrias Venoco CA
US Dept of the Interior
Congorhar Inc.
Ametek Inc.
Waldemar S. Nelson & Company, Inc.
Henkel Corp.
Drexelbrook Engineering Company
INERIS
DMT
Weidmuller Inc.
Pittsburgh Research Laboratory
Mobil Chemical Company
Servomex Co.
Appleton Electric
R. Stahl, Inc.
Intertek Testing Services
Motorola Inc.
PCS Engineering
Bartec US Corp.
ABB Kent-Taylor Inc.
Huntsman Corp.
U. S. Coast Guard
Bailey Fischer & Porter
Pepperl + Fuchs
O-Z/Gedney
Hawke America
Shanghai Institute of Process Automation Instr.
Phoenix Mecano
Underwriters Laboratories, Inc.
Honeywell Inc.
Symbol Technologies Inc.
Killark
Turck Inc.
Factory Mutual Research Corporation
Appleton Electric
Applied Automation Inc.
Factory Mutual Research Corp.
Feltronics Corp.
Consultant
Drexelbrook Engineering Company
Bailey Controls Company
Dexion House
Petromarine of Texas
Hawke America
Detector Electronics Corp.
3M Company
Data Instruments
Pepperl + Fuchs Inc.
ERDCO Engineering Corp.
5
M. Oakes
*E. Olson
C. Oudar
A. Page III
R. Pellizze
J. Propst
C. Sandberg
C. Sawyer
J. Shaffer
T. Schnaare
W. Shao
A. Stafford
D. Stevens
*D. Styrcula
J. Thomason
P. Thurnherr
L. Truscott
*P. Turner
T. Vu
D. Wechsler
R. Weinzler
____________________________
WAGO Corp.
3M Company
ExLoc Corp.
MSHA Certification Center
Intertek Testing Services
Shell Development Company
Raychem Corp.
MI Cable Co Inc.
Endress+Hauser Co.
Rosemount, Inc.
Canadian Standards Association
Foxboro Company
Chevron USA Inc.
Underwriters Laboratories Inc.
Omni Industrial Systems Inc.
Thuba Ltd.
Motorola Inc.
3M Company
Milltronics Ltd.
Union Carbide Corp.
Consultant
* One vote per company
This standard was approved for publication by the ISA Standards and Practices Board
on November 30, 1997.
NAME
COMPANY
R. Webb, Vice President
H. Baumann
D. Bishop
P. Brett
W. Calder III
M. Cohen
H. Dammeyer
R. Dieck
W. Holland
H. Hopkins
A. Iverson
K. Lindner
V. Maggioli
T. McAvinew
A. McCauley, Jr.
G. McFarland
E. Montgomery
D. Rapley
R. Reimer
J. Rennie
W. Weidman
J. Weiss
J. Whetstone
Pacific Gas & Electric Company
H. D. Baumann & Associates, Ltd.
Chevron USA Production Company
Honeywell, Inc.
Calder Enterprises
Flexonics, Inc.
Ohio State University
Pratt & Whitney
Southern Company Services, Inc.
Consultant
Ivy Optiks
Endress + Hauser GmbH
Feltronics Corp.
Instrumentation & Control Engineering Services
Chagrin Valley Controls, Inc.
Honeywell Inc.
Fluor Daniel, Inc.
Rapley Engineering Services
Rockwell Automation
Factory Mutual Research Corporation
Parsons Energy and Chemical Group
Electric Power Research Institute
National Institute of Standards & Technology
6
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
M. Widmeyer,
H.R. Wiegle
C. Williams
G. Wood
M. Zielinski
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
Carnegie-Mellon University
Canus Corp.
Eastman Kodak Company
Graeme Wood Consulting
Fisher•Rosemount Systems, Inc.
7
Contents
Foreword.................................................................................................................................... 10
1 General................................................................................................................................... 10
1.1 Scope ..................................................................................................................... 10
1.2 Normative References .......................................................................................... 11
1.3 Purpose................................................................................................................... 12
2 Definitions and terms ........................................................................................................... 12
2.10 Explosive limits ..................................................................................................... 17
3 Safety and area classification.............................................................................................. 19
3.1 Safety principles .................................................................................................... 19
3.2 Area classification objectives............................................................................... 19
4 Area classification procedure.............................................................................................. 20
4.1 General.................................................................................................................... 20
4.2 Sources of release ................................................................................................. 20
4.3 Type of Zone ........................................................................................................... 22
4.4 Extent of Zone ........................................................................................................ 22
4.5 Extent of zone — General comments................................................................... 25
5 Ventilation.............................................................................................................................. 25
5.1 General.................................................................................................................... 25
5.2 Main types of ventilation ....................................................................................... 25
5.3 Degree of ventilation.............................................................................................. 26
5.4 Availability of ventilation ....................................................................................... 26
6 Documentation...................................................................................................................... 26
6.1 General.................................................................................................................... 26
6.2 Drawings, data sheets and tables ........................................................................ 27
Annex A (Informative) – Examples of sources of release .................................................... 29
A.1 Process plants and facilities ................................................................................ 29
A.2 Openings ................................................................................................................ 29
Annex B –
B.1
B.2
B.3
B.4
(Informative) Ventilation ........................................................................................ 33
Natural ventilation ................................................................................................. 33
Artificial ventilation ............................................................................................... 34
Degree of ventilation ............................................................................................. 35
Assessment of degree of ventilation and its influence on the hazardous
area ........................................................................................................................ 35
B.5 Availability of ventilation ...................................................................................... 39
B.6 Practical guide ....................................................................................................... 40
B.7 Calculations to ascertain the degree of ventilation ........................................... 41
Annex C (Informative) – Examples of hazardous area classification ................................. 49
Annex D (Informative) – Use of combustible gas detection equipment .............................. 73
Annex E (Informative) – United States major deviations ...................................................... 75
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
9
Foreword
This recommended practice is a modification of IEC 79-10, Electrical Apparatus for Explosive
Gas Atmospheres, Part 10: Classification of Hazardous Areas, normalized as an American
National Standard, with additional material added as appendices specifically for the classification
of locations for electrical installations classified as Class I, Zone 0, Zone 1, or Zone 2
Classification of hazardous areas. U.S. National Deviations are shown by strikeout through text
deleted and underline under text added. There are five annexes in this recommended practice.
All annexes are Informative and are not considered part of this recommended practice.
1 General
1.1 Scope
This part of IEC 79 ISA-RP12.24.01 is concerned with the classification of hazardous areas
where flammable gas or vapor risks may arise, in order to permit the proper selection and
installation of apparatus for use in such hazardous areas (see Notes 1 and 4).
It is intended to be applied where there may be a risk of ignition due to the presence of flammable
gas or vapor, mixed with air under normal atmospheric conditions (see Note 2), but it does not
apply to:
a)
b)
c)
d)
mines susceptible to firedamp;
the processing and manufacture of explosives;
areas where a risk may arise due to the presence of ignitable dusts or fibers;
catastrophic failures which are beyond the concept of abnormality dealt with in this
standard recommended practice (see Note 3);
e) rooms used for medical purposes;
f) areas where the presence of flammable mist may give rise to an unpredictable risk and
which require special consideration (see Note 5).
This standard recommended practice does not take into account the effects of consequential
damage.
Definitions and explanations of terms are given together with the main principles and procedures
relating to hazardous area classification.
For detailed recommendations regarding the extent of the hazardous areas in specific industries
or applications, reference may be made to the codes, standards, or recommended practices
relating to those industries or applications.
10
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
NOTES—
1) For the purpose of this standard recommended practice, an area is a three-dimensional
region or space.
2) Atmospheric conditions include variations above and below reference levels of 101.3 kPa
(1.013 mbar) and 20 °C (293 K), provided that the variations have a negligible effect on
the explosion properties of the flammable materials.
3) Catastrophic failure in this context is applied, for example, to the rupture of a process
vessel or pipeline, and such other events that are not predictable.
4) In any process plant facility , irrespective of size, there may be numerous sources of
ignition apart from those associated with electrical apparatus. Appropriate precautions
will be necessary to ensure safety in this context. This standard recommended practice
may be used with judgment for other ignition sources.
5) Mists may form or be present at the same time as flammable vapors. This may affect the
way flammable material disperses and the extent of any hazardous areas. The strict
application of area classification for gases and vapors may not be appropriate because
the flammability characteristics of mists are not always predictable. Whilst it can be
difficult to decide upon the type and extent of zones, the criteria applicable to gases and
vapors will, in most cases, give a safe result. However, special consideration should
always be given to the danger of ignition of flammable mists.
1.2 Normative References
The following normative documents contain provisions which, through reference in this text,
constitute provisions of this part of IEC 79 recommended practice. At the time of publication, the
editions indicated were valid. All normative documents are subject to revision, and parties to
agreements based on this part of IEC 79 recommended practice are encouraged to investigate
the possibility of applying the most recent editions of the normative documents indicated below.
Members of IEC and ISO maintain registers of currently valid International Standards. ANSI
maintains registers of currently valid American National Standards.
ANSI/ISA-S12.13, Part I, Performance Requirements, Combustible Gas Detectors
ANSI/ISA- RP12.13, Part II, Installation Operation and Maintenance of Combustible Gas
Detection Instruments
IEC 50(426): 1990, International Electrotechnical Vocabulary (IEV) - Chapter 426.- Electrical
apparatus for explosive atmospheres
IEC 79-1A: Construction and Verification Tests of Flameproof Enclosures of Electrical
Apparatus
IEC 79-3: Spark-Test Apparatus For Intrinsically Safe Circuits
IEC 79-4: 1975, Electrical apparatus for explosive gas atmospheres - Part 4: Method of test for
ignition temperature
IEC 79-4A: 1970, First supplement to IEC 79-4 (1966)
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
11
IEC 79-12: Classification of Mixtures of Gases or Vapours With Air According To Their Maximum
Experimental Safe Gaps and Minimum Igniting Currents
ANSI/API RP 14F: Recommended Practice for Design and Installation of Electrical Systems for
Offshore Production Platforms
API RP 505: Recommended Practice for Classification of Locations for Electrical Installations at
Petroleum Facilities Classified as Class I, Zone 0, Zone 1, or Zone 2
ASTM E-659: Test for Autoignition Temperature of Liquid Chemicals
IP 15, Institute of Petroleum: Area Classification Code for Petroleum Installations – Part 15
NFPA 30: Flammable and Combustible Liquids Code
NFPA 70: National Electrical Code®
NFPA 325M: Guide to Fire Hazard Properties of Flammable Liquids, Gases, and Volatile Solids
NFPA 497: Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified)
Locations For Electrical Installations In Chemical Process Areas
1.3 Purpose
The purpose of this recommended practice is to provide guidelines for classifying hazardous
(classified) locations for the selection and installation of electrical equipment. Basic definitions
given in the 1995 edition of IEC 79-10, Electrical apparatus for explosive gas atmospheres,
Classification of hazardous areas, have been followed in developing this recommended practice.
This publication is only a guide and requires the application of sound engineering judgment.
Electrical installations in areas where flammable liquids or gases are produced, processed,
stored or otherwise handled can be suitably designed if the locations of potential sources of
release and accumulation are clearly defined. Once a location has been classified, requirements
for electrical equipment and associated wiring should be determined from applicable
publications. Applicable publications may include NFPA No. 70 (NEC) or API RP 14F. Reference
Section 1.2 for other possible applicable publications.
2 Definitions and terms
For the purpose of this recommended practice part of IEC 79, the following definitions and terms
apply.
2.1
explosive gas atmosphere: A mixture with air, under atmospheric conditions, of a
flammable material in the form of gas or vapor in which, after ignition, combustion spreads
throughout the unconsumed mixture. [IEV 426-02-03, modified]
12
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
NOTE—Although a mixture which has a concentration above the upper explosive limit (UEL)
is not an explosive gas atmosphere, it can readily become so and in certain cases, for area
classification purposes, it is advisable to consider it as an explosive gas atmosphere.
2.2
hazardous area: An area in which an explosive gas atmosphere is present, or may be
expected to be present, in quantities such as to require special precautions for the construction,
installation and use of apparatus. [IEV 426-03-01, modified]
NOTE —The term “hazardous (classified) location” is defined in ANSI/ISA-S12.1.01.
RP12.24.01 uses the term “hazardous area.” The two terms should be considered
synonymous.
2.3
non-hazardous area: An area in which an explosive gas atmosphere is not expected to
be present in quantities such as to require special precautions for the construction, installation
and use of apparatus. [IEV 426-03-02, modified]
NOTE—The term “unclassified location” is defined in ANSI/ISA S12.1.01. S12.24.01 uses
the term “non-hazardous area.” The two terms should be considered synonymous.
2.4
Zone 0, 1, and 2 locations zone: Hazardous areas are classified into zones based upon
the frequency of the occurrence and duration of an explosive gas atmosphere as follows:
2.4.1 class I, zone 0: An area in which an explosive gas atmosphere is present continuously or
for long periods. [IEV 426-03-03, modified]
2.4.2 class I, Zone 1: An area in which an explosive gas atmosphere is likely to occur in normal
operation. [IEV 426-03-04]
2.4.3 class I, zone 2: An area in which an explosive gas atmosphere is not likely to occur in
normal operation and, if it does occur, is likely to do so only infrequently and will exist for a short
period only. [IEV 426-03-05, modified]
2.4.1 Class I, Zone 0. A Class I, Zone 0, location is a location (1) in which ignitable
concentrations of flammable gases or vapors are present continuously; or (2) in which ignitable
concentrations of flammable gases or vapors are present for long periods of time.
NOTES—
1)
As a guide in determining when flammable gases or vapors are present continuously or
for long periods of time, refer to Recommended Practice for Classification of Locations for
Electrical Installations of Petroleum Facilities Classified as Class I, Zone 0, Zone 1, or
Zone 2, API RP 505-1996; Electrical Apparatus for Explosive Gas Atmospheres,
Classifications of Hazardous Areas, IEC 79-10; and Area Classification Code for
Petroleum Installations, Model Code, Part 15, Institute of Petroleum; and Electrical
Apparatus for Explosive Gas Atmospheres, Classifications of Hazardous (Classified)
Locations, ISA S12.24.01-1997.
2)
This classification includes locations inside vented tanks or vessels containing volatile
flammable liquids; inside inadequately vented spraying or coating enclosures, where
volatile flammable solvents are used; between the inner and outer roof sections of a
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
13
floating roof tank containing volatile flammable liquids; inside open vessels, tanks and pits
containing volatile flammable liquids; the interior of an exhaust duct that is used to vent
ignitable concentrations of gases or vapors; and inside inadequately ventilated
enclosures containing normally venting instruments utilizing or analyzing flammable fluids
and venting to the inside of the enclosures.
3)
It is not good practice to install electrical equipment in Zone 0 locations except when the
equipment is essential to the process or when other locations are not feasible. (See NEC
Section 505-2 FPN No. 2.) If it is necessary to install electrical systems in a Zone 0
location, it is good practice to install intrinsically safe systems as described by NEC
Article 504.
2.4.2 Class I, Zone 1. A Class I, Zone 1 location is a location (1) in which ignitable
concentrations of flammable gases or vapors are likely to exist under normal operating
conditions; or (2) in which ignitable concentrations of flammable gases or vapors may exist
frequently because of repair or maintenance operations or because of leakage; or (3) in which
equipment is operated or processes are carried on, of such a nature that equipment breakdown
or faulty operations could result in the release of ignitable concentrations of flammable gases or
vapors and also cause simultaneous failure of electrical equipment in a mode to cause the
electrical equipment to become a source of ignition; or (4) that is adjacent to a Class I, Zone 0
location from which ignitable concentrations of vapors could be communicated, unless
communication is prevented by adequate positive pressure ventilation from a source of clean air
and effective safeguards against ventilation failure are provided.
NOTES—
1)
Normal operations is considered the situation when facility equipment is operating within
its design parameters. Minor releases of flammable material may be part of normal
operations. Minor releases include the releases from mechanical packings on pumps.
Failures that involve repair or shutdown (such as the breakdown of pump seats and flange
gaskets, and spillage caused by accidents) are not considered normal operation.
2)
This classification usually includes locations where volatile flammable liquids or liquefied
flammable gases are transferred from one container to another. In areas in the vicinity of
spraying and painting operations where flammable solvents are used; adequately
ventilated drying rooms or compartments for evaporation of flammable solvents;
adequately ventilated locations containing fat and oil extraction equipment using volatile
flammable solvents; portions of cleaning and dyeing plants where volatile flammable
liquids are used; adequately ventilated gas generator rooms and other portions of gas
manufacturing facilities where flammable gas may escape; inadequately ventilated pump
rooms for flammable gas or for volatile flammable liquids; the interiors of refrigerators and
freezers in which volatile flammable materials are stored in the open, lightly stoppered, or
easily ruptured containers; and other locations where ignitable concentrations of
flammable vapors or gases are likely to occur in the course of normal operation, but not
classified Zone 0.
2.4.3 Class I, Zone 2. A Class I, Zone 2 location is a location (1) in which ignitable
concentrations of flammable gases or vapors are not likely to occur in normal operation and if
they do occur will exist only for a short period; or (2) in which volatile flammable liquids,
flammable gases, or flammable vapors are handled, processed, or used, but in which the liquids,
14
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
gases, or vapors normally are confined within closed containers of closed systems from which
they can escape, only as a result of accidental rupture or breakdown of the containers or system,
or as the result of the abnormal operation of the equipment with which the liquids or gases are
handled, processed, or used; or (3) in which ignitable concentrations of flammable gases or
vapors normally are prevented by positive mechanical ventilation, but which may become
hazardous as a result of failure or abnormal operation of the ventilation equipment; or (4) that is
adjacent to a Class I, Zone 1 location, from which ignitable concentrations of flammable gases or
vapors could be communicated, unless such communication is prevented by adequate positivepressure ventilation from a source of clean air, and effective safeguards against ventilation failure
are provided.
NOTES—
1) The Zone 2 classification usually includes locations where volatile flammable liquids or
flammable gases or vapors are used, but which would become hazardous only in case of
an accident or of some unusual operating condition.
2) Indications of the frequency of the occurrence and duration may be taken from codes
relating to specific industries or applications IP-15, The Institute of Petroleum Area
Classification Code for Petroleum Installations, Part 15; API RP 505, Recommended
Practice for Classification of Locations for Electrical Installations at Petroleum Facilities
Classified as Class I, Zone 0, Zone 1, or Zone 2; and NFPA 497, Classification of
Flammable Liquids, Gasses, or Vapors and of Hazardous (Classified) Locations For
Electrical Installations In Chemical Process Areas.
2.4.4 Grouping and Classification. For purposes of testing, approval, and area classification,
various air mixtures (not oxygen enriched) shall be grouped as follows (NFPA 70 Mod):
Group I is intended for use in describing atmospheres containing firedamp (a mixture of gases,
composed mostly of methane, found underground, usually in mines). This standard does not
apply to installations underground in mines.
Group II shall be subdivided into IIC, IIB, and IIA, as noted below, according to the nature of the
gas or vapor, for protection techniques “d,” “ia,” “ib” [ia], and [ib], and, where applicable, “n” and
“o.”
Class I, Zone combustible materials are divided into three groups (NFPA 497):
Group IIC - Atmospheres containing acetylene, hydrogen, or flammable gas, flammable liquid
produced vapor, or combustible liquid produced vapor mixed with air that may burn or explode,
having either a maximum experimental safe gap (MESG) value less than or equal to 0.50 mm or
a minimum igniting current ratio (MIC ratio) less than 0.45.
Group IIB- Atmospheres containing acetaldehyde, ethylene, or flammable gas, flammable liquid
produced vapor, or combustible liquid produced vapor mixed with air that may burn or explode
having either a maximum experimental safe gap (MESG) value greater than 0.50 mm and less
than or equal to 0.90 mm or a minimum igniting current ratio (MIC ratio) greater than 0.45 and
less than or equal to 0.80.
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
15
Group IIA - Atmospheres containing acetone, ammonia, ethyl alcohol, gasoline, methane,
propane, or flammable gas, flammable liquid produced vapor, or combustible liquid produced
vapor mixed with air that may burn or explode having either a maximum experimental safe gap
(MESG) value greater than 0.90 mm or a minimum igniting current ratio (MIC ratio) greater than
0.80.
NOTES—
1)
The gas and vapor sub-division as described above is based on the maximum
experimental safe gap (MESG), minimum igniting current (MIC), or both. Test equipment
for determining the MESG is described in Construction and Verification Tests of
Flameproof Enclosures of Electrical Apparatus, IEC 79-1A (Amendment No. 1) and UL
Technical Report No. 58. The test equipment for determining MIC is described in SparkTest Apparatus For Intrinsically-Safe Circuits, IEC 79-3 . The classification of gases or
vapors according to their maximum experimental safe gaps and minimum igniting
currents is described in Classification of Mixtures of Gases or Vapours With Air According
To Their Maximum Experimental Safe Gaps and Minimum Igniting Currents, IEC 79-12 .
2)
Verification of electrical equipment utilizing protection techniques “e,” “m,” “p,” and “q,”
due to design technique, does not require tests involving MESG or MIC. Therefore,
Group II is not required to be sub-divided for these protection techniques.
3)
It is necessary that the meanings of the different equipment markings and Group II
classifications be carefully observed to avoid confusion with Class I, Divisions 1 and 2,
Groups A, B, C, and D.
2.5
source of release: A point or location from which a flammable gas, vapor, or liquid may
be released into the atmosphere such that an explosive gas atmosphere could be formed. [IEV
426-03-06, modified]
2.6
grades of release: There are three basic grades of release, as listed below in order of
decreasing likelihood of the explosive gas atmosphere being present:
a) continuous grade;
b) primary grade;
c) secondary grade.
A source of release may give rise to any one of these grades of release, or to a combination of
more than one.
2.6.1 continuous grade of release: A release which is continuous or is expected to occur for
long periods.
2.6.2 primary grade of release: A release which can be expected to occur periodically or
occasionally during normal operation.
2.6.3 secondary grade of release: A release which is not expected to occur in normal
operation and if it does occur, is likely to do so only infrequently and for short periods.
16
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
2.7
release rate: The quantity of flammable gas or vapor emitted per unit time from the
source of release.
2.8
normal operation: The situation when the equipment is operating within its design
parameters.
NOTES—
1) Minor releases of flammable material may be part of normal operation. For example,
releases from seals which rely on wetting by the fluid which is being pumped are
considered to be minor releases.
2) Failures (such as the breakdown of pump seals, flange gaskets or spillages caused by
accidents) which involve urgent repair or shut-down are not considered to be part of
normal operation.
3)
Unless otherwise specified, normal operation for motors is assumed to be rated full-load
steady conditions.
2.9
ventilation: Movement of air and its replacement with fresh air due to the effects of wind,
temperature gradients, or artificial means (for example, fans or extractors).
2.10 Explosive limits
2.10.1 lower explosive limit (LEL): The concentration of flammable gas or vapor in air, below
which the gas atmosphere is not explosive. [IEV 426-02-09, modified].
2.10.2 upper explosive limit (UEL): The concentration of flammable gas or vapor in air, above
which the gas atmosphere is non-explosive. [IEV 426-02-10, modified]
NOTE — For the purpose of this standard. the terms 'explosive' and 'flammable' should be
considered synonymous. The term “explosive” as it relates to atmospheres and mixtures is in
the process of being replaced with the more technically correct “flammable” throughout many
national and international standards. This document, however, continues to use the term
“explosive” in many of the definitions that are repeated here as those definitions have been
derived from documents that have yet to be updated.
2.11 relative density of a gas or a vapor: The density of a gas or a vapor relative to the
density of air at the same pressure and at the same temperature (air is equal to 1.0).
2.12 flammable material: A material which is flammable of itself, or is capable of producing a
flammable gas, vapor or mist.
2.13 flammable liquid (Class IA, IB, and IC): A liquid having a flash point below 100°F
(37.8°C) and having a Reid vapor pressure not exceeding 40 pounds per square inch absolute
(2068.6 mm Hg or 276 kilopascals) at 100°F (37.8°C). Flammable (Class I) liquids are subdivided
into Classes IA, IB, and IC. (Reference NFPA No. 30). A liquid capable of producing a flammable
vapor under any foreseeable operating conditions.
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
17
2.14 flammable gas or vapor: Gas or vapor which, when mixed with air in certain proportions,
will form an explosive gas atmosphere.
2.15 flammable mist: Droplets of flammable liquid, dispersed in air so as to form an explosive
atmosphere.
2.16 flashpoint: The lowest liquid temperature at which, under certain standardized
conditions, a liquid gives off vapors in a quantity such as to be capable of forming an ignitable
vapor/air mixture. [IEV 426-02-141]
2.17 boiling point: The temperature of a liquid boiling at an ambient pressure of 101.3 kPa
(1.013 mbar).
NOTE —For liquid mixtures, the initial boiling point should be used. Initial boiling point is
used for liquid mixtures to indicate the lowest value of the boiling point for the range of liquids
present, as determined in a standard laboratory distillation without fractionation.
2.18 vapor pressure: The pressure exerted when a solid or liquid is in equilibrium with its own
vapor. It is a function of the substance and of the temperature.
2.19 Ignition temperature of an explosive gas atmosphere: The lowest temperature of a
heated surface at which, under specified conditions, the ignition of a flammable substance in the
form of a gas or vapor mixture with air will occur.
NOTE—IEC 79-4 and IEC 79-4A standardize a method for the determination of this
temperature. [IEV 426-02-01, modified). To determine properties of specific flammable
liquids, flammable gases and volatile solids, refer to ASTM E-659, Test for Autoignition
Temperature of Liquid Chemicals. To obtain the autoignition temperature of specific
flammable liquids, flammable gases, and volatile solids, refer to NFPA 497.
2.20 Combustible Liquid (Class II, IIIA, and IIIB): A liquid having a flash point at or above
100°F (37.8°C).
Combustible Liquids are subdivided as follows:
Class II liquids, those having flash points at or above 100°F (37.8°C) and below 140°F
(60°C).
Class IIIA liquids, those having flash points at or above 140°F (60°C) and below 200°F
(93°C).
Class IIIB liquids, those having flash points at or above 200°F (93°C).
2.21 Flammable Highly Volatile Liquids (HVLs): Liquids whose Reid vapor pressure
exceeds 40 pounds per square inch absolute (2068.6 mm Hg or 276 kPa) at 100°F (37.8°C).
2.22 Location: Throughout this recommended practice, reference is made to areas, spaces,
and locations. These terms should be considered interchangeable terms designating a threedimensional space.
18
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
3 Safety and area classification
3.1 Safety principles
Installations in which flammable materials are handled or stored should be designed, operated
and maintained so that any releases of flammable material, and consequently the extent of
hazardous areas, are kept to a minimum, whether in normal operation or otherwise, with regard
to frequency, duration and quantity.
In the case of maintenance activities other than those of normal operation, the extent of the zone
may be affected, but it is expected that this would be dealt with by a permit-to-work system.
In emergency situations, reliance should be placed on the isolation of unsuitable electrical
equipment, shut-down of the process, isolation of process vessels, containment of spillages and,
if possible, the provision of additional emergency ventilation.
In a situation in which there may be an explosive gas atmosphere, the following steps should be
taken:
a) eliminate the likelihood of an explosive gas atmosphere occurring around the source of
ignition, or
b)
eliminate the source of ignition.
Where this is not possible, protective measures, process equipment, systems and procedures
should be selected and prepared so the likelihood of the coincidence of a) and b) is so small as to
be acceptable. Such measures may be used singly if they are recognized as being highly
reliable, or in combination to achieve an equivalent level of safety.
3.2 Area classification objectives
Area classification is a method of analyzing and classifying the environment where explosive gas
atmospheres may occur so as to facilitate the proper selection and installation of apparatus to be
used safely in that environment, taking into account gas groups and temperature classes.
In most practical situations where flammable materials are used, it is difficult to ensure that an
explosive gas atmosphere will never occur. It may also be difficult to ensure that apparatus will
never give rise to a source of ignition. Therefore, in situations where an explosive gas
atmosphere has a high likelihood of occurring, reliance is placed on using apparatus which has a
low likelihood of creating a source of ignition. Conversely, where the likelihood of an explosive
gas atmosphere occurring is reduced, apparatus constructed to a less rigorous standard may be
used.
It is rarely possible by a simple examination of a facility plant or facility plant design to decide
which parts of the facility plant can be equated to the three zonal definitions (Class I, Zones 0, 1
and 2). A more detailed approach is therefore necessary, and this involves the analysis of the
basic possibility of an explosive gas atmosphere occurring.
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
19
The first step is to assess the likelihood of this, in accordance with the definitions of Class I, Zone
0, Zone 1 and Zone 2. Once the likely frequency and duration of release (and hence the grade of
release), the release rate, concentration, velocity, ventilation and other factors which affect the
type and/or or extent of the zone, or both, have been determined, there is then a firm basis on
which to determine the likely presence of an explosive gas atmosphere in the surrounding areas.
This approach therefore requires detailed consideration to be given to each item of process
equipment which contains a flammable material, and which could therefore be a source of
release.
In particular, Class I, Zone 0 or Zone 1 areas should be minimized in number and extent by
design or suitable operating procedures. In other words, facilities plants and installations shall
should be mainly Class I, Zone 2 or non-hazardous. Where release of flammable material is
unavoidable, process equipment items should be limited to those which give secondary grade
releases or, failing this (that is where primary or continuous grade releases are unavoidable), the
releases should be of very limited quantity and rate. In carrying out area classification, these
principles should receive prime consideration. Where necessary, the design, operation and
location of process equipment should ensure that, even when it is operating abnormally, the
amount of flammable material released into the atmosphere is minimized, so as to reduce the
extent of the hazardous area.
Once a facility plant has been classified and all necessary records made, it is important that no
modification to equipment or operating procedures is made without discussion with those
responsible for the area classification. Unauthorized action may invalidate the area classification.
It is necessary to ensure that all equipment affecting the area classification which has been
subjected to maintenance is carefully checked during and after re-assembly to ensure that the
integrity of the original design, as it affects safety, has been maintained before it is returned to
service.
4 Area classification procedure
4.1 General
The area classification should be carried out by those who have knowledge of the properties of
flammable materials, the process and the equipment, in consultation, as appropriate, with safety,
electrical and other engineering personnel.
The following subclauses give guidance on the procedure for classifying areas in which there
may be an explosive atmosphere and on the extent of Class I, Zones 0, 1 and 2. An example of a
schematic approach to the classification of hazardous areas is given in figure C.1.
4.2 Sources of release
The basic elements for establishing the hazardous zone types are the identification of the source
of release and the determination of the grade of release.
Since an explosive-gas atmosphere can exist only if a flammable gas or vapor is present with air,
it is necessary to decide if any of these flammable materials can exist in the area concerned.
20
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
Generally speaking, such gases and vapors (and flammable liquids and solids which may give
rise to them) are contained within process equipment which may or may not be totally enclosed.
It is necessary to identify where a flammable atmosphere can exist inside a process facility plant,
or where a release of flammable materials can create a flammable atmosphere outside a process
facility plant.
Each item of process equipment (for example, tank, pump, pipeline, or vessel, etc.) should be
considered as a potential source of release of flammable material. If the item cannot contain
flammable material it clearly will not necessitate will clearly not give rise to a hazardous area
around it. The same will apply if the item contains a flammable material but cannot release it into
the atmosphere (for example, an all-welded pipeline is not considered to be a source of release).
4.2.1 Experience has shown that certain locations may be unclassified regardless of the
ventilation rate since the occurrence of flammable gas or vapor liberation from some apparatus is
so infrequent. Examples of such locations include the following:
a) Locations where flammable substances are contained in:
1) all-welded closed piping systems without valves, flanges or similar devices, or
2) continuous metallic tubing without valves, fittings, flanges, or similar devices.
b) Locations where flammable liquids, gases or vapors are transported or stored in certain
containers or vessels (Refer to NFPA recommendations and Department of
Transportation (DOT) regulations specifying containers for flammable liquids and gases).
4.2.2 Adequately ventilated locations surrounding equipment that has continuous flame sources
(e.g., unprotected fired vessels and flare tips) need not be classified solely by reason of the fuel
gas being considered as a source of release for area classification purposes.
NOTES —
1) It may be prudent to classify portions of these locations. For example, electrical
equipment may be exposed to flammable gas during a purge cycle of a fired heater or
furnace.
2) The lack of classification around unprotected fired vessels and flare tips does not imply
the safe placement of fired vessels and flare tips in the proximity to other sources of
release because unprotected fired vessels and flare tips are themselves sources of
ignition.
4.2.3 Other locations may be non-hazardous depending on the degree of ventilation and other
factors.
If it is established that the item may release flammable material into the atmosphere, it is
necessary, first of all, to determine the grade of release in accordance with the definitions, by
establishing the likely frequency and duration of the release. It should be recognized that the
opening-up of parts of enclosed process systems (for example, during filter changing or batch
filling) should also be considered as sources of release when developing the area classification.
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
21
By means of this procedure, each release will be graded either “continuous,” “primary” or
“secondary.”
Having established the grade of the release, it is necessary to determine the release rate and
other factors which may influence the type and extent of the zone.
4.3 Type of Zone
The likelihood of the presence of an explosive gas atmosphere, and hence the type of zone,
depends mainly on the grade of release and the ventilation.
NOTE —A continuous grade of release normally leads to a Class I, Zone 0, a primary grade
to Class I, Zone 1 and a secondary grade to Class I, Zone 2 (see Annex B).
4.4 Extent of Zone
The extent of the zone is mainly affected by the following chemical and physical parameters,
some of which are intrinsic properties of the flammable material; others are specific to the
process. For simplicity, the effect of each parameter listed below assumes that the other
parameters remain unchanged.
4.4.1 Release rate of gas or vapor
The greater the release rate the larger the extent of the zone. The release rate depends itself on
other parameters, namely:
a) Geometry of the source of release
This is related to the physical characteristics of the source of release -- for example, an
open surface, leaking flange, etc. (see Annex A).
b) Release velocity
For a given source of release, the release rate increases with the release velocity. In the
case of a product contained within process equipment, the release velocity is related to
the process pressure and the geometry of the source of release. The size of a cloud of
flammable gas or vapor is determined by the rate of flammable vapor release and the rate
of dispersion. Gas and vapor flowing from a leak at high velocity will develop a
coneshaped jet which will entrain air and be self-diluting. The extent of the explosive
atmosphere will be almost independent of wind velocity. If the release is at low velocity or
if its velocity is destroyed by impingement on a solid object, it will be carried by the wind
and its dilution and extent will depend on wind velocity.
c) Concentration
The release rate increases with the concentration of flammable vapor or gas in the
released mixture.
22
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
d) Volatility of a flammable liquid
The volatility of a flammable liquid is related principally to the vapor pressure, and the
heat of vaporization. If the vapor pressure is not known, the boiling point and flashpoint
can be used as a guide.
An explosive atmosphere cannot exist if the flashpoint is above the relevant maximum
temperature of the flammable liquid. The lower the flashpoint, the greater may be the
extent of the zone. If a flammable material is released in a way that forms a mist (for
example, by spraying) an explosive atmosphere may be formed below the flashpoint of
the material for example.
NOTES—
1) Flashpoints of flammable liquids are not precise physical quantities, particularly where
mixtures are involved.
2) Some liquids (for example, certain halogenated hydrocarbons) do not possess a
flashpoint, although they are capable of producing an explosive gas atmosphere. In
these cases, the equilibrium liquid temperature which corresponds to the saturated
concentration at the lower explosive limit should be compared with the relevant maximum
liquid temperature.
e) Liquid temperature
The vapor pressure increases with temperature, thus increasing the release rate due to
evaporation.
NOTE — The temperature of the liquid after it has been released may be increased —for
example, by a hot surface or by a high ambient temperature.
4.4.2 Lower explosive limit (LEL)
For a given release volume, the lower the LEL the greater will be the extent of the zone.
4.4.3 Ventilation
With increased ventilation, the extent of the zone will be reduced. Obstacles which impede the
ventilation may increase the extent of the zone. On the other hand, some obstacles (for example,
dikes, walls or ceilings) may limit the extent.
4.4.4 Relative density of the gas or vapor when it is released
If the gas or vapor is significantly lighter than air, it will tend to move upwards. If significantly
heavier, it will tend to accumulate at ground level. The horizontal extent of the zone at ground
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
23
level will increase with increasing relative density, and the vertical extent above the source will
increase with decreasing relative density.
NOTES—
1) For practical applications, a gas or vapor which has a relative density below 0.8 is
regarded as being lighter than air. If the relative density is above 1.2 it is regarded as
being heavier than air. Between these values, both of these possibilities should be
considered.
2) Experience has shown that ammonia is hard to ignite and a gas release will dissipate
rapidly in the open air, so any explosive gas atmosphere will be of negligible extent.
4.4.5 Other parameters to be considered
a) Climatic conditions.
b) Topography.
4.4.6 Illustrative examples
Some ways in which the above-mentioned parameters affect the vapor or gas release rate, and
hence the extent of the zone, are demonstrated in the examples in Annex C.
a) Source of release: open surface of liquid
In most cases, the liquid temperature will be below the boiling point and the vapor release
rate will depend principally on the following parameters:
- liquid temperature;
- vapor pressure of the liquid at its surface temperature; and
- dimensions of the evaporation surface.
b)
Source of release: virtually instantaneous evaporation of a liquid (for example, from a jet
or spray)
Since the discharged liquid vaporizes virtually instantaneously, the vapor release rate is equal
to the liquid flow rate, and this depends on the following parameters:
-
liquid pressure; and
geometry of the source of release.
Where the liquid is not instantaneously vaporized, the situation is complex because droplets,
liquid jets and pools may create separate sources of release.
c) Source of release: leakage of a gas mixture
The gas release rate is affected by the following parameters:
-
24
pressure within the equipment which contains the gas;
geometry of the source of release; and
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
-
concentration of flammable gas in the released mixture.
For examples of sources of release see clause A.2.
4.5 Extent of zone — General comments
4.5.1 Consideration should always be given to the possibility that a gas which is heavier than
air may flow into areas below ground level (for example, pits or depressions) and that a gas that is
lighter than air may be retained at high level (for example, in a roof space).
4.5.2 Where the source of release is situated outside an area or in an adjoining area, the
penetration of a significant quantity of flammable gas or vapor into the area can be prevented by
suitable means such as:
a) physical barriers;
b) maintaining a static overpressure in the area relative to the adjacent hazardous areas, so
preventing the ingress of the hazardous atmosphere;
c) purging the area with a significant flow of air, so ensuring that the air escapes from all
openings where the hazardous gas or vapor may enter.
5 Ventilation
5.1 General
Gas or vapor released into the atmosphere can be diluted by dispersion or diffusion into the air
until its concentration is below the lower explosion limit. Ventilation, i.e., air movement leading to
replacement of the atmosphere in a (hypothetical) volume around the source of release by fresh
air, will promote dispersion. Suitable ventilation rates can also avoid persistence of an explosive
gas atmosphere, thus influencing the type of zone.
5.2 Main types of ventilation
Ventilation can be accomplished by the movement of air due to natural means such as the wind
and and/or by temperature gradients or by artificial means such as fans. So two main types of
ventilation are thus recognized:
a) natural ventilation;
b) artificial ventilation, general or local.
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
25
5.3 Degree of ventilation
The most important factor is that the degree or amount of ventilation is directly related to the
types of sources of release and their corresponding release rates. This is irrespective of the type
of ventilation, whether it be wind speed or the number of air changes per time unit. Thus, optimal
ventilation conditions in the hazardous area can be achieved, and the higher the amount of
ventilation in respect of the possible release rates, the smaller will be the extent of the zones
(hazardous areas), in some cases reducing them to a negligible extent (nonhazardous area).
Practical examples for guidance on the degree of ventilation to be used are given in Annex B.
5.4 Availability of ventilation
The availability of ventilation has an influence on the presence or formation of an explosive
atmosphere and thus also on the type of zone. Guidance on availability is given in Annex B.
NOTE — Combining the concepts of degree of ventilation and level of availability results in a
quantitative method for the evaluation of zone type (see Annex B).
6 Documentation
6.1 General
It is recommended that area classification is undertaken in such a way that the various steps
which lead to the final area classification are properly documented.
All relevant information used should be referred to. Examples of such information, or of a method
used, would be:
a) recommendations from relevant recommended practices, codes, and standards;
b) gas and vapor dispersion characteristics and calculations;
c) a study of ventilation characteristics in relation to flammable material release parameters
so that the effectiveness of the ventilation can be evaluated.
The results of the area classification study and any subsequent alterations to it should shall be
placed on record.
Those properties which are relevant to area classification of all process materials used at the
facility on the should be listed and should include flashpoint, boiling point, ignition temperature,
vapor pressure, vapor density, explosive limits, gas group and temperature class. An example is
given in Tables C.1 and C.2.
26
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
6.2 Drawings, data sheets and tables
Area classification documents should include plans and elevations, as appropriate, which show
both the type and extent of zones, ignition temperature and, hence, temperature class and gas
group.
Where the topography of an area influences the extent of the zones, this should be documented.
The documents should also include other relevant information such as:
a) the location and identification of sources of release. For large and complex plants
facilities or process areas, it may be helpful to itemize or number the sources of release
so as to facilitate cross-referencing between the area classification data sheets and the
drawings;
b) the position of openings in buildings (for example, doors, windows, and inlets and outlets
of air for ventilation).
The area classification symbols which are shown in Figure C.2 are the preferred ones, but
alternatives may be used provided that they are clearly defined in the documents.
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
27
Annex A (Informative) – Examples of sources of release
A.1 Process plants and facilities
The following examples are not intended to be rigidly applied and may need to be varied to suit
particular process equipment and situations.
A.1.1 Sources giving a continuous grade of release:
a) the surface of a flammable liquid in a fixed roof tank, with a permanent vent to the
atmosphere;
b) the surface of a flammable liquid which is open to the atmosphere continuously or for long
periods (for example, an oil-water separator).
A.1.2 Sources giving a primary grade of release:
a) seals of pumps, compressors or valves if release of flammable material during normal
operation is expected;
b) water drainage points on vessels which contain flammable liquids, which may release
flammable material into the atmosphere while draining off water during normal operation;
c) sample points which are expected to release flammable material into the atmosphere
during normal operation;
d) relief valves, vents and other openings which are expected to release flammable material
into the atmosphere during normal operation.
A.1.3 Sources giving a secondary grade of release:
a) seals of pumps, compressors and valves where release of flammable material during
normal operation of the equipment is not expected;
b) flanges, connections and pipe fittings, where release of flammable material is not
expected during normal operation;
c) sample points which are not expected to release flammable material during normal
operation;
d) relief valves, vents and other openings which are not expected to release flammable
material into the atmosphere during normal operation.
A.2 Openings
The following examples are not intended to be rigidly applied, but may need to be varied to suit
particular situations.
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
29
A.2.1 Openings as possible sources of release
Openings between areas should be considered as possible sources of release. The grade of
release will depend upon:
•
the zone type of the adjoining area;
•
the frequency and duration of opening periods;
•
the effectiveness of seals or joints;
•
the difference in pressure between the areas involved.
A.2.2 Openings classification
Openings are classified as A, B, C, D with the following characteristics:
A.2.2.1 Type A Openings- Openings not conforming to the characteristics specified for types B,
C or D.
Examples:
•
open passages for access or utilities – for example, ducts, pipes through walls, ceilings
and floors;
•
fixed ventilation outlets in rooms, buildings and similar openings of types B, C and D
which are opened frequently or for long periods.
A.2.2.2 Type B Openings - Openings that are normally closed (for example, automatic closing)
and infrequently opened, and which are close-fitting.
A.2.2.3 Type C Openings - Openings normally closed and infrequently opened, conforming to
type B, which are also fitted with sealing devices (for example, a gasket) along the whole
perimeter; or two type B openings type B in series, having independent automatic closing
devices.
A.2.2.4 Type D Openings - Openings normally closed conforming to type C which can only be
opened by special means or in an emergency.
Type D openings are effectively sealed, such as in utility passages (for example, ducts and pipes)
or can be a combination of one type C opening type C adjacent to a hazardous area and one
type B opening type B in series.
30
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
Table A.1 - Effect of openings on grade of release
Zone upstream of opening
Class I, Zone 0
Class I, Zone 1
Class I, Zone 2
Opening type
Grade of release of openings
considered as
sources of release
A
Continuous
B
(Continuous)/primary
C
Secondary
D
No release
A
Primary
B
(Primary)/secondary
C
(Secondary)/no release
D
No release
A
Secondary
B
(Secondary)/no release
C
No release
D
No release
Note — For grades of release shown in brackets, the frequency of operation of the openings should be
considered in the design.
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
31
Annex B (Informative) — Ventilation
Introduction
The purpose of this annex is to assess the degree of ventilation and to extend clause 5 by
defining ventilation conditions and by means of explanations, examples and calculation, so giving
guidance on the design of artificial ventilation systems, since these are of paramount importance
in the control of the dispersion of releases of flammable gases and vapors.
The methods developed allow the determination of the type of zone by:
•
estimating the minimum ventilation rate required to prevent significant build-up of an
explosive atmosphere and using this to calculate a hypothetical volume, V2, which, with
an estimated dispersion time, t, allows determination of the degree of ventilation. It is not
intended that these calculations be used to determine the extent of the hazardous areas;
•
determining the type of zone from the degree and availability of ventilation and the grade
of release.
Although primarily of direct use in indoor situations, the concepts explained may assist in outdoor
locations – for example, by determination of the application of table B.1.
B.1 Natural ventilation
This is a type of ventilation which is accomplished by the movement of air caused by the wind, by
temperature gradients, or by both. In open air situations, natural ventilation will often be sufficient
to ensure dispersal of any explosive atmosphere which arises in the area. Natural ventilation
may also be effective in certain indoor situations (for example, where a building has openings in
its walls or and/or roof).
NOTE —For outdoor areas the evaluation of ventilation should normally be based on an
assumed minimum wind speed of 0.5 m/s, which will be present virtually continuously. The
wind speed will frequently be above 2 m/s.
Examples of natural ventilation:
•
open air situations typical of those in the chemical and petroleum industries – for
example, open structures, pipe racks, pump bays and the like;
•
an open building which, having regard to the relative density of the gases or and/or
vapors involved, has openings in the walls, roof, or both, so dimensioned and located that
the ventilation inside the building, for the purpose of area classification, can be regarded
as equivalent to that in an open air situation;
•
a building which is not an open building but which has natural ventilation (generally less
than that of an open building) provided by permanent openings made for ventilation
purposes.
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
33
B.2 Artificial ventilation
The air movement required for ventilation is provided by artificial means – for example, fans or
extractors. Although artificial ventilation is mainly applied inside a room or enclosed space, it can
also be applied to situations in the open air to compensate for restricted or impeded natural
ventilation due to obstacles.
The artificial ventilation of an area may be either general or local and, for both of these, differing
degrees of air movement and replacement can be appropriate.
With the use of artificial ventilation it is possible to achieve:
•
reduction in the extent of zones;
•
shortening of the time of persistence of an explosive atmosphere;
•
prevention of the generation of an explosive atmosphere.
Artificial ventilation makes it possible to provide an effective and reliable ventilation system in an
indoor situation. An artificial ventilation system which is designed for explosion protection should
meet the following requirements:
•
its effectiveness should be controlled and monitored;
•
consideration should be given to the classification immediately outside the extract
exhaust system discharge point;
•
for ventilation of a hazardous area, the ventilation air normally should be drawn from a
non-hazardous area;
•
before determining the dimensions and design of the ventilation system, the location,
grade of release and release rate should be defined.
In addition, the following factors will influence the quality of an artificial ventilation system:
•
flammable gases and vapors usually have densities other than that of air; thus they will
tend to accumulate near to either the floor or ceiling of an enclosed area, where air
movement is likely to be reduced;
•
changes in gas density with temperature;
•
impediments and obstacles may cause reduced, or even no air movement – i.e., no
ventilation in certain parts of the area.
Examples of general artificial ventilation:
34
•
a building which is provided with fans in the walls, roof, or both to improve the general
ventilation in the building;
•
an open air situation provided with suitably located fans to improve the general ventilation
of the area.
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
Examples of local artificial ventilation:
•
an air/vapor extraction system applied to an item of process equipment which
continuously or periodically releases flammable vapor;
•
a forced or extract ventilation system applied to a small, ventilated local area where it is
expected that an explosive atmosphere may otherwise occur.
B.3 Degree of ventilation
The effectiveness of the ventilation in controlling dispersion and persistence of the explosive
atmosphere will depend upon the degree and availability of ventilation and the design of the
system. For example, ventilation may not be sufficient to prevent the formation of an explosive
atmosphere, but may be sufficient to avoid persistence of an explosive atmosphere.
The following three degrees of ventilation are recognized:
B.3.1 High ventilation (VH)
Can reduce the concentration at the source of release virtually instantaneously, resulting in a
concentration below the lower explosive limit. A zone of small (even negligible) extent results.
B.3.2 Medium ventilation (VM)
Can control the concentration, resulting in a stable situation in which the concentration beyond
the zone boundary is below the LEL whilst release is in progress and where the explosive
atmosphere does not persist unduly after release has stopped.
The extent and type of zone are limited to the design parameters.
B.3.3 Low ventilation (VL)
Cannot control the concentration whilst release is in progress or and/or cannot prevent undue
persistence of a flammable atmosphere after release has stopped.
B.4 Assessment of degree of ventilation and its influence on the hazardous
area
The size of a cloud of flammable gas or vapor and the time for which it persists after release
stops can be controlled by means of ventilation. A method for evaluating the degree of ventilation
required to control the extent and persistence of an explosive atmosphere is described below.
It should be appreciated that the method is subject to the limitations described and therefore
gives only approximate results. The use of the safety factors should, however, ensure that the
results obtained err on the side of safety. The application of the method is illustrated by a number
of hypothetical examples.
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
35
The assessment of the degree of ventilation first requires the knowledge of the maximum release
rate of gas or vapor at the source of release, either by verified experience, reasonable calculation
or sound assumptions.
Estimation of hypothetical volume Vz
The theoretical minimum ventilation flow rate to dilute a given release of flammable material to
the required concentration below the lower explosive limit can be calculated by means of the
formula:
(dV / dt )min =
(dG / dt )max
k x LEL
x
T
293
(B.1)
where
is the minimum volumetric flowrate of fresh air (volume per time, m3/s);
(dV/dt)min
(dG/dt)max, is the maximum rate of release at source (mass per time, kg/s);
LEL
is the lower explosive limit (mass per volume, kg/m3);
k
is a safety factor applied to the LEL; typically:
k = 0.25 (continuous and primary grades of release); and
k = 0.5 (secondary grades of release).
T
is the ambient temperature (in kelvins).
NOTE — For converting LEL (vol %) to LEL (kg/m3), the following formula may be used
for normal atmospheric conditions as given in 1.1:
LEL (kg/m3) = 0.416 x 10-3 x M x LEL (vol %)
where M is the molecular mass (kg/kmol).
With a given number of air changes per unit time, C, related to the general ventilation of the area,
a hypothetical volume Vz of potentially explosive atmosphere around the source of release can
be estimated using the following formula:
Vz =
(dV / dt )
min
(B.2)
C
where
C is the number of fresh air changes per unit time (s-1);
36
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
Formula (B.2) would hold for an instantaneous and homogeneous mixing at the source of release
given ideal flow conditions of the fresh air. In practice, such ideal situations will generally not be
found – for example, because of possible impediments to the air flow, resulting in badly ventilated
parts of the area. Thus, the effective air exchange at the source of release will be lower than that
given by C in formula (B.4), leading to an increased volume Vz. By introducing an additional
correction (quality) factor, f, to formula (B.2), one obtains:
Vz =
f x (dV / dt )min
C
(B.3)
where f denotes the efficiency of the ventilation in terms of its effectiveness in diluting the
explosive atmosphere, with f ranging from f = 1 (ideal situation) to, typically, f = 5 (impeded air
flow).
The volume Vz represents the volume over which the mean concentration of flammable gas or
vapor will be either 0.25 or 0.5 times the LEL, depending on the value of the safety factor, k, used
in formula (B.2) (B.1). This means that, at the extremities of the hypothetical volume estimated,
the concentration of gas or vapor will be significantly below the LEL, i.e. the hypothetical volume
where the concentration is above the LEL would be less than Vz.
Enclosed area
For an enclosed area, C is given by:
C=
dVtot / dt
Vo
(B.4)
where
dVtot /dt
is the total flow rate of fresh air, and
VO
is the total volume being ventilated.
Open air
In an open air situation, even very low wind speeds will create a high number of air changes. For
example, consider a hypothetical cube with the dimensions of a few meters in an open area. In
this case a wind speed of approximately 0.5 m/s will provide an air exchange rate of more than
100/h (0.03/s).
In a conservative approximation using C = 0.03/s for an open air situation, a hypothetical volume
Vz of potentially explosive atmosphere can be obtained by using formula (B.5):
Vz =
(dV / dt )
min
0.03
(B.5)
where
dV/dt
is in volume units per second, and
0.03
is the number of air changes per second.
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
37
However, because of the different dispersion mechanism, this method will generally result in an
overlarge volume. Dispersion is normally more rapid in an open air situation.
Estimation of persistence time t
The time (t) required for the average concentration to fall from an initial value Xo to the LEL times
k after the release has stopped can be estimated from:
t=
−f
LEL x k
ln
C
Xo
(B.6)
where
Xo
is the initial concentration of the flammable substance measured in the same units as the
LEL (i.e., % vol or kg/m3). Somewhere in the explosive atmosphere, the concentration of
the flammable may be 100% vol (in general, only in the very close vicinity of the release
source). However, when calculating t, the proper value for Xo to be taken depends on the
particular case, considering among others the affected volume as well as the frequency
and the duration of the release, and for most practical cases it seems reasonable to take
a concentration above LEL for Xo;
C
is the number of air changes per unit time;
t
is in the same time units as C, i.e., if C is the number of air changes per second, then the
time t will be in seconds;
f
is a factor to allow for imperfect mixing (see formula B.3). It varies from 5 (for example, for
ventilation with air entering through cracks and a single exhaust opening), to about 1 (for
example, for ventilation with air entering through a perforated coiling and multiple
exhausts);
Ln
is the natural logarithm (i.e., 2.303 log10);
k
is a safety factor related to the LEL – see formula (B.2) (B.1).
The numerical value of t obtained by equation (B.6) by itself does not constitute a quantitative
means of deciding on the zone type. It provides additional information that has to be compared
with the time scale of the particular process and situation.
Estimation of degree of ventilation
A continuous grade of release normally leads to a Class I, Zone 0, a primary grade to Class I,
Zone 1 and a secondary grade to Class I, Zone 2. This may not always be true because of the
effect of ventilation.
In some cases, the degree and level of availability of ventilation may be so high that in practice
there is no hazardous area. Alternatively, the degree of ventilation may be so low that the resulting
zone has a lower zone number (i.e., a Class I, Zone I hazardous area from a secondary grade
38
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
source). This occurs, for example, when the level of ventilation is such that the explosive
atmosphere persists and is dispersed only slowly after the gas or vapor release has stopped.
Thus, the explosive atmosphere persists for longer than would be expected for the grade of release.
The volume Vz can be used to provide a means of rating the ventilation as high, medium or low.
The persistence time t can be used to decide what degree of ventilation is required for one area
to comply with the definitions of Class I, Zones 0, I or 2.
The ventilation may be regarded as high (VH) when the volume Vz is very small or even
negligible. With the ventilation in operation, the source of release can be regarded as not
producing an explosive atmosphere, i.e., the surrounding area is non-hazardous. However, there
will be an explosive atmosphere, albeit of negligible extent, close to the source of release.
In practice, high ventilation can generally be applied only to a local artificial ventilation system
around a source, to small enclosed areas, or to very low release rates. Firstly, most enclosed
areas contain multiple sources of release. It is not good practice to have multiple small
hazardous areas within an area generally classified as non-hazardous. Secondly, with the typical
release rates considered for area classification, natural ventilation is often insufficient even in the
open. Furthermore, it is normally impracticable to ventilate artificially larger enclosed areas at
the rates required.
The volume Vz does not give any indication of the time for which the explosive atmosphere would
persist after release has stopped. This is not relevant in the high ventilation (VH) case, but is a
factor in assessing if the ventilation is medium (VM) or low (VL).
Ventilation regarded as medium (VM) should control the dispersion of the release of flammable
vapor or gas. The time taken to disperse an explosive atmosphere after release has stopped
should be such that the condition for either a Class I, Zone 1 or Class I, Zone 2 is met, depending
on whether the grade of release is primary or secondary. The acceptable dispersion time
depends on the expected frequency of release and the duration of each release. The volume Vz
will often be less than the volume of any enclosed area. In this case it may be acceptable to
classify only part of the enclosed area as hazardous. In some cases, depending on the size of
the enclosed area, the volume Vz can be similar to the enclosed volume. In this case, all of the
enclosed area should be classified as hazardous.
If the zonal concept is not met, then the ventilation should be regarded as low (VL). With low
ventilation, the volume Vz will often be similar to or greater than the volume of any enclosed area.
Low ventilation (VL) should not generally occur in open air situations except where there are
restrictions to air flow (for example, in pits).
B.5 Availability of ventilation
The availability of ventilation has an influence on the presence or formation of an explosive
atmosphere. Thus the availability (as well as the degree) of ventilation needs to be taken into
consideration when determining the type of zone.
Three levels of availability of the ventilation should be considered (see examples in Annex C):
•
good: ventilation is present virtually continuously;
•
fair: ventilation is expected to be present during normal operation. Discontinuities are
permitted provided they occur infrequently and for short periods;
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
39
•
poor: ventilation which does not meet the standard of fair or good, but discontinuities are
not expected to occur for long periods.
Ventilation that does not even meet the requirement for “poor” availability must not be considered
to contribute to the ventilation of the area.
Natural Ventilation
For outdoor areas the evaluation of ventilation should normally be based on an assumed
minimum wind speed of 0.5 m/s, which will be present virtually continuously. In which case the
availability of the ventilation can be considered as “good.”
Artificial Ventilation
In assessing the availability of artificial ventilation, the reliability of the equipment and the
availability of, for example, standby blowers should be considered. Good availability will normally
require, on failure, automatic start-up of standby blower(s). However, if provision is made for
preventing the release of flammable material when the ventilation has failed (for example, by
automatically closing down the process), the classification determined with the ventilation
operating need not be modified, i.e., the availability may be assumed to be good.
B.6 Practical guide
The effect of ventilation on the type of the zones can be summarized in Table B.1. Some
calculations are included in B.7.
Table B.1 - Influence of ventilation on type of zone
Grade
Ventilation
Degree
of
Release
Continuous
High
Medium
Good
Fair
Poor
(Zone 0 NE)
Nonhazardous1)
(Zone 0 NE)
(Zone 0 NE)
Zone 21)
(Zone 1 NE)
Zone 21)
Zone 11)
(Zone 1 NE)
Zone 21)
Availability
Good
Zone 0
Fair
Zone 0 +
Zone 2
Low
Poor
Zone 0 +
Zone 1
Good, Fair
or Poor
Zone 0
Zone 1
Zone 1 +
Zone 1 +
Zone 1 or
(Zone 1 NE)
Zone 2
Zone 2
NonZone 0 3)
1)
hazardous
2)
(Zone 2 NE) (Zone 2 NE) Zone 2
Zone 2
Zone 2
Zone 2
Zone 1
Secondary
NonNonand even
hazardous1) hazardous1)
Zone 03)
1) Zone 0 NE, 1 NE or 2 NE indicates a theoretical zone that would be of negligible extent under normal conditions.
Primary
2) The Zone 2 area created by a secondary grade of release may exceed that attributable to a primary or continuous
grade of release; in which case, the greater distance should be taken.
3) Will be Zone 0 if the ventilation is so weak and the release is such that in practice an explosive atmosphere exists
virtually continuously (i.e., approaching a “no ventilation” condition).
NOTE —“+” signifies “surrounded by.”
“Zone 0,” “Zone 1,” and “Zone 2” are understood to be preceded by “Class I.”
40
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
B.7 Calculations to ascertain the degree of ventilation
Calculation No. 1
Characteristics of release
Flammable material
toluene vapor
Source of release
flange
Lower explosion limit (LEL)
0.046 kg/m3 (1.2 % vol.)
Grade of release
continuous
Safety factor, k
0.25
Release rate, (dG/dt)max
2.8 x 10-10 kg/s
Ventilation characteristics
Indoor situation
Number of air changes, C
1/h, (2.8 x 10-4 / s)
Quality factor, f
5
Ambient temperature, T
20 °C (293 K)
Temperature coefficient, (T/293 K)
1
Minimum volumetric flow rate of fresh air:
(dV / dt )
min
=
(dG / dt )
max
k x LEL
x
2.8 x 10 −10 293
T
x
=
= 2.4 x 10 −8 m 3 / s
293 0.25 x 0.046 293
Evaluation of hypothetical volume Vz:
Vz =
f x (dV / dt )min
C
=
5 x 2 .4 x 10 −8
2 .8 x 10 −4
= 4.3 x10 −4 m 3
Time of persistence:
This is not applicable to a continuous release.
Conclusion
The hypothetical volume Vz, is reduced to a negligible value.
The degree of ventilation is considered as high with regard to the source.
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
41
Calculation No. 2
Characteristics of release
Flammable material
toluene vapor
Source of release
failure of flange
Lower explosion limit (LEL)
0.046 kg/m3 (1.2% vol.)
Grade of release
secondary
Safety factor, k
0.5
Release rate, (dG/dt)max
2.8 x 10-6 kg/s
Ventilation characteristics
Indoor situation
Number of air changes, C
1/h (2.8 x 10-4 / s)
Quality factor, f
5
Ambient temperature, T
20 °C (293 K)
Temperature coefficient, (T/293 K)
1
Minimum volumetric flow rate of fresh air:
(dV / dt )min
=
(dG / dt )max
k x LIE
x
2 .8 x 10 −6 293
T
x
=
= 1.2 x 10 −4 m 3 / s
293 0.5 x 0.046 293
Evaluation of hypothetical volume Vz:
Vz =
f x (dV / dt )min
C
=
5 x 1.2 x10 −4
2 .8 x10 −4
= 2 .2 m 3
Time of persistence:
t=
LEL x k −5 1.2 x 0.5
−f
ln
ln
=
= 25.6 h
1
100
C
X0
Conclusion
The hypothetical volume Vz is significant but can be controlled.
The degree of ventilation is considered as medium with regard to the source on this basis.
However any release would persist and the concept of Class I, Zone 2 may not be met.
42
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
Calculation No. 3
Characteristics of release
Flammable material
propane gas
Source of release
can-filling nozzle
Lower explosion limit (LEL)
0.039 kg/m3 (2.1 % vol.)
Grade of release
primary
Safety factor, k
0.25
Release rate, (dG/dt)max
0.005 kg/s
Ventilation characteristics
Indoor situation
Number of air changes, C
20/h (45.6 x 10-3 / s)
Quality factor, f
1
Ambient temperature, T
35° C (308 K)
Temperature coefficient, (T/293 K)
1.05
Minimum volumetric flow rate of fresh air:
(dV / dt )min
=
(dG / dt )max
k x LEL
x
0.005
308
T
x
=
= 0.6 m 3 / s
293 0.25 x 0.039 293
Evaluation of hypothetical volume Vz:
Vz =
f x (dV / dt )min
C
=
1 x 0.6
5.6 x 10 −3
. x 10 2 m 3
= 11
Time of persistence:
t=
LEL x k −1 2 .1 x 0.25
−f
ln
ln
=
= 0.26 h
20
100
C
Xo
Conclusion
The hypothetical volume Vz is significant, but can be controlled.
The degree of ventilation is considered as medium with regard to the source based on this
criterion. With a persistence time of 0.26 h, the concept of Class I, Zone 1 may not be met if the
operation is repeated frequently.
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
43
Calculation No. 4
Characteristics of release
Flammable material
ammonia gas
Source of release
evaporator valve
Lower explosion limit (LEL)
0.105 kg/m3 (14.8 % vol.)
Grade of release
secondary
Safety factor, k
0.5
Release rate, (dG/dt)max
5 x 10-6 kg/s
Ventilation characteristics
Indoor situation
Number of air changes, C
15/h, (4.2 x 10-3/s)
Quality factor, f
1
Ambient temperature, T
20° C (293 K)
Temperature coefficient, (T/293 K)
1
Minimum volumetric flow rate of fresh air:
(dV / dt )min
=
(dG / dt )max
k x LEL
x
5 x 10 −6
293
T
x
=
= 9 .5 x 10 −5 m 3 / s
293 0.5 x 0.105 293
Estimation of hypothetical volume Vz:
Vz =
f x (dV / dt )min
C
=
1 x 9.5 x 10 −5
4 .2 x 10 −3
= 0.02 m 3
Time of persistence:
t=
LEL x k −1 14.8 x 0.5
−f
ln
ln
=
= 0.17 h (10 min)
15
100
C
Xo
Conclusion
The hypothetical volume Vz is reduced to a negligible value.
The degree of ventilation is considered as high with regard to the source. However any
equipment located adjacent to the valve should be suitable for Class I, Zone 2 (see table B.1).
44
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
Calculation No. 5
Characteristics of release
Flammable material
propane gas
Source of release
compressor seal
Lower explosion limit (LEL)
0.039 kg/m3 (2.1 % vol.)
Grade of release
secondary
Safety factor, k
0.5
Release rate, (dG/dt)max
0.02 kg/s
Ventilation characteristics
Indoor situation
Number of air changes, C
2/h, (5.6 x 10-4/s)
Quality factor, f
5
Ambient temperature, T
20° C (293 K)
Temperature coefficient, (T/293 K)
1
Minimum volumetric flow rate of fresh air:
(dV / dt )min
=
(dG / dt )max
k x LEL
x
0.02
293
T
x
=
= 1.02 m 3 / s
293 0.5 x 0.039 293
Estimation of hypothetical volume Vz:
Vz =
f x (dV / dt )min
C
=
5 x 1.02
5.6 x 10 −4
= 9 ,200 m 3
Time of persistence
t=
LEL x k −5 2 .1 x 0.5
−f
ln
ln
=
= 11.4 h
2
100
C
Xo
Conclusion
In a room of 10 m x 15 m x 6 m for example, the hypothetical volume Vz would extend beyond the
physical boundaries and would persist. The degree of ventilation is considered as low with
regard to the source.
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
45
Calculation No. 6
Characteristics of release
Flammable material
methane gas
Source of release
pipe fitting
Lower explosion limit (LEL)
0.033 kg/m3 (5 % Vol)
Grade of release
secondary
Safety factor, k
0.5
Release rate, (dG/dt)max
1 kg/s
Ventilation characteristics
Outdoor situation
Minimum wind speed
0.5 ms
Resulting in an air exchange, C
>3 x 10-2 / s
Quality factor, f
3
Ambient temperature, T
15° C (288 K)
Temperature coefficient, (T/293 K)
0.98
Minimum volumetric flow rate of fresh air.
(dV / dt )min
=
(dG / dt )max
k x LEL
x
1
T
x=
= 59 .3 m 3 / s
293
0.5 x 0.033
Estimation of hypothetical volume Vz:
Vz
f x (dV / dt )min
C
=
3 x 59 .3
3 x 10 −2
= 5 900 m 3
Time of persistence:
t=
5 x 0.5
LEL x k
−f
−3
ln
ln
=
= 370 s (max imum)
0.03
100
C
Xo
Conclusion
The hypothetical volume Vz is significant but can be controlled, and would not persist.
The degree of ventilation is considered as medium with regard to the source.
46
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
Calculation No. 7
Characteristics of release
Flammable material
toluene vapor
Source of release
failure of flange
Lower explosion limit (LEL)
0.046 kg/m3 (1.2 % vol.)
Grade of release
secondary
Safety factor, k
0.5
Release rate, (dG/dt)max
6 x 10-4 kg/s
Ventilation characteristics
Indoor situation
Number of air changes, C
12/h (3.33 x 10-3)
Quality factor
2
Ambient temperature, T
20° C (293 K)
Temperature coefficient, (T/293 K)
1
Minimum volumetric flow rate of fresh air:
(dV / dt )min
=
(dG / dt )max
k x LEL
x
6 x 10 −4
293
T
x
=
= 26 x 10 −3 m 3 / s
293 0.5 x 0.046 293
Evaluation of hypothetical volume Vz:
Time of persistence:
t=
LEL x k −2 1.2 x 0.5
−f
ln
ln
=
= 0.85 h (51 min )
12
100
C
Xo
Conclusion
The hypothetical volume Vz is significant but can be controlled.
The degree of ventilation is considered as medium with regard to the source. Based on this
persistence time, the concept of Class I, Zone 2 would be met.
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
47
Annex C (Informative) – Examples of hazardous area classification
NOTES—
1) The contents of this annex, extracted from IEC 79-10 with minor editorial changes, are
not entirely applicable to classifications made in the United States, but are included as an
informative annex since they contain valuable information. The National Electrical Code
(NFPA 70) requires a Zone 1 "transition zone" between a Zone 0 location and a Zone 2
location; similarly, it requires a Zone 2 "transition zone" between a Zone 1 location and an
unclassified location. IEC 79-10 does not require these "transition zones.” Reference the
definitions shown in 2.4.1 through 2.4.3, which show the NEC definitions of Zones 0, 1,
and 2 as added (underlined) material and the IEC 79-10 definitions of Zones 0, 1, and 2
as deleted (struck-through) material. The user of this document should be aware of the
differences and adjust classifications made in accordance with NFPA 70.
2) A vaportight barrier can be used to confine or prevent the spreading of gas or vapor. If a
vaportight barrier is used to isolate a classified location, it is not necessary to have a
transition zone and the other side of the barrier can be unclassified. Also, adequate
positive-pressure ventilation from a source of clean air can be used to eliminate the
transition zone if effective safeguards against ventilation failure are provided.
The practice of area classification involves a knowledge of the behavior of flammable
C.1
gases and liquids when they are released from containment, and sound engineering judgment
based on experience of the performance of items of facility plant equipment under specified
conditions. For this reason, it is not practicable to give every conceivable variation of facility
plant and process characteristics. Therefore, the examples chosen are those which best
describe the overall philosophy of area classification, so as to permit the safe use of apparatus in
hazardous locations, where the dangerous material is a flammable liquid, liquefied gas or vapor,
or material which is normally gaseous and flammable when mixed with air in appropriate
concentrations.
In arriving at the distances shown in the diagrams, specific facility plant component
C.2
conditions have been given. The leakage conditions have been considered in relation to the
mechanical performance of the equipment and other representative design criteria. They are not
generally applicable; factors such as inventory of process material, shut-off time, dispersion time,
pressure, temperature and other criteria related both to facility plant components and process
material all affect the area classification and will need to be applied to the particular problem
being considered. Thus these examples represent guidance only and will need to be adapted so
as to take into account particular circumstances.
According to the national or industrial code, standard, or recommended practice selected,
C.3
the shape and extent of the zones may vary.
The intention of the examples which follow is not primarily that they should be used for
C.4
area classification. Their principal objective is to demonstrate typical results which might be
obtained in practice in a number of different situations by following the guidance and procedures
in this recommended practice standard. They may also be of use in developing detailed
supplementary standards.
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
49
The figures shown are taken from, or correspond closely to, those in various national or
C.5
industrial codes. They are intended only as a guide to the magnitude of the zones; in individual
cases, the extent and shape of the zones may be taken from the relevant code.
If it is intended that the examples given in this recommended practice standard be used
C.6
for area classification in practice, account must be taken of the specific details of each individual
case.
In each example, some, but not all, of the parameters which influence the type and extent
C.7
of zones are given. The result of the classification normally gives a conservative result, taking
into account those factors which have been specified and others which it has been possible to
identify but not quantify. This means that, if it is possible to specify the operating parameters
more closely, a more precise classification will be obtained.
50
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
Example No. 1
A normal industrial pump mounted at ground level, situated outdoors, pumping flammable liquid:
Not to scale
Principal factors which influence the type and extent of zones
Plant and process
Ventilation
Type …………
Degree ………
Availability ….
Natural
Medium
Poor
Source of release
Grade of release
Pump seal …
Artificial
High*
Fair
Primary and secondary
Product
Flash point
Below process and ambient temperature
Vapour Density
Greater than air
* Airflow from pump motor.
Taking into account relevant parameters, the following are typical values which will be obtained
for a pump having a capacity of 50 m3/h and operating at a low pressure:
a=3 m horizontally from source of release;
b=1 m from ground level and up to 1 m above the source of release.
NOTE —Due to the high air flow, the extent of Zone 1 is negligible.
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
51
Example No. 2
A normal industrial pump mounted at ground level, situated indoors, pumping flammable liquid:
Not to scale
Principal factors which influence the type and extent of zones
Plant and process
Ventilation
Type …………
Degree ………
Availability ….
Artificial
Medium
Fair
Source of release
Grade of release
Pump seal (packed gland)
and pool at floor level …
Primary and secondary
Product
Flash point
Below process and ambient temperature
Vapour Density
Greater than air
Taking into account relevant parameters, the following are typical values which will be obtained
for a pump having a capacity of 50 m3/h and operating at a low pressure:
a=1.5 m horizontally from source of release;
b=1 m from ground level and up to 1 m above the source of release;
c=3 m horizontally from source of release.
52
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
Example No. 3
Pressure breathing valve in the open air, from process vessel:
Not to scale
Principal factors which influence the type and extent of zones
Facility and process
Ventilation
Type …………
Degree ………
Availability ….
Source of release
Natural
Medium
Fair
Grade of release
Outlet from valve ….. Primary
Product
Gasoline
Gas Density …….
Greater than air
Taking into account relevant parameters, the following are typical values which will be obtained
for a valve where the opening pressure of the valve is approximately 0.15 Mpa (1.5 bar):
a=3 m horizontally from source of release;
b=5 m from ground level and up to 1 m above the source of release.
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
53
Example No. 4
Control valve, installed in a closed process pipework system conveying flammable gas:
Not to scale
Principal factors which influence the type and extent of zones
Facility and process
Ventilation
Type …………
Degree ………
Availability ….
Source of release
Natural
Medium
Fair
Grade of release
Valve shaft seal …… Secondary
Product
Gas ….
Propane
Gas Density ….
Greater than air
Taking into account relevant parameters, the following are typical values which will be obtained
for the example:
a=1 m horizontally from source of release;
54
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
Example No. 5
A fixed process mixing vessel, situated indoors, being operated regularly for operational reasons.
The liquids are piped into and out of the vessel through all welded pipework flanged at the vessel:
Not to scale
Principal factors which influence the type and extent of zones
Facility and process
Ventilation
Type …………
Degree ………
Availability ….
Artificial
Low inside the vessel
Fair
Source of release
Grade of release
Liquid surface within the vessel
The opening in the vessel
Spillage or leakage of liquid close to the vessel
Continuous
Primary
Secondary
Product
Flash point
Vapour Density
Below process and ambient temperature
Greater than air
Taking into account relevant parameters, the following are typical values which will be obtained
for this example:
a=1 m horizontally from source of release;
b=1 m above the source of release;
c=1 m horizontally;
d=2 m horizontally;
e=1 m above ground.
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
55
Example No. 6
Oil/water gravity separator, situated outdoors, open to the atmosphere, in a petroleum refinery:
Not to scale
Principal factors which influence the type and extent of zones
Facility and process
Ventilation
Type …………
Degree ………
Availability ….
Natural
Medium
Poor
Source of release
Grade of release
Liquid surface …..
Process disturbances …
Continuous
Secondary
Product
Flash point
Below process and ambient temperature
Vapour Density
Greater than air
Taking into account relevant parameters, the following are typical values which will be obtained
for this example.
a=3 m horizontally from the separator;
b=1 m above ground level;
c=7.5m horizontally;
d=3 m above ground level.
56
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
Example No. 7
Hydrogen compressor in a building which is open at ground level.
Not to scale
Principal factors which influence the type and extent of zones
Facility and process
Ventilation
Type …………
Degree ………
Availability ….
Natural
Medium
Good
Source of release
Compressor seals, valves and flanges
close to the compressor …….
Product
Gas …
Gas Density
Grade of release
Secondary
Hydrogen
Lighter than air
Taking into account relevant parameters, the following are typical values which will be obtained
for this example:
a=3 m horizontally from source of release;
b=1 m horizontally from ventilating openings;
c=1 m above ventilation openings.
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
57
Example No. 8
Flammable liquid storage tank, situated outdoors, with fixed roof and no internal floating roof:
Not to scale
Principal factors which influence the type and extent of zones
Facility and process
Ventilation
Type …………
Degree ………
Availability ….
Natural
Medium*
Good
Source of release
Grade of release
Liquid surface ……
Vent opening and other openings in the roof …
Flanges, etc. inside dike bund and overfilling of the tank ….
Continuous
Primary
Secondary
Product
Flash point
Below process and ambient temperature
Vapour Density
Greater than air
Taking into account relevant parameters, the following are typical values which will be obtained
for this example:
a=3 m from vent openings;
b=3 m above the roof;
c=3 m horizontally from the tank.
58
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
Example No. 9
Tank car filling installation, situated outdoors, for gasoline, top filling:
Not to scale
Principal factors which influence the type and extent of zones
Facility and process
Ventilation
Type …………
Degree ………
Availability ….
Natural
Medium
Poor
Source of release
Opening in tank roof ……
Spillage at ground level …
Grade of release
Primary
Secondary
Product
Flash point
Vapour Density
Below process and ambient temperature
Greater than air
Taking into account relevant parameters, the following are typical values which will be obtained
for this example:
a=1.5 m horizontally from source of release;
b= horizontally to island (gantry) boundary;
c=1.5 m above source of release;
d=1 m above ground level;
e=4.5 m horizontally from drainage channel;
f=1.5 m horizontally from Zone 1;
g=1.0 m above Zone 1.
NOTE —If the system is a closed system with vapour recovery, the distances can be
reduced, such that Zone 1 may be of negligible extent and Zone 2 significantly reduced.
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
59
Example No. 10
Mixing room in a paint factory:
Not to scale
This example shows one way of using the individual examples Nos. 2 and 5. In this simplified
example, four paint-mixing vessels (item 2) are situated in one room. There are also three
pumps (item 1) for liquid in the same room.
Principal factors which influence the type and extent of zones are given in the tables in examples
Nos. 2 and 5.
Taking into account relevant parameters, (see hazardous area classification data sheets), the
following are typical values which will be obtained for this example:
a=2 m;
b=4 m;
c=3 m;
d=1.5 m.
The drawing No. 10 is a plan view, for vertical extent of the zones see examples Nos. 2 and 5.
NOTE—As in examples Nos. 2 and 5, the zones have a cylindrical shape around the sources
of release. However, in practice, the zones are usually increased to a box shape if the
vessels are situated close to each other. In this way there are no unclassified small pockets.
It is assumed that the pumps and vessels are connected by all-welded pipework and that flanges,
valves, etc. are located close to these items of equipment.
In practice, there may be other sources of release in the room, for example open vessels, but
these have not been taken into account in this example.
If the room is small, it is recommended that Zone 2 extends to the limits of the room.
60
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
61
Hazardous area classification data sheet – Part I: Flammable material flat and characteristics
Sheet 1 of 2
Plant: paint factory (example 10)
1
No.
2
Name
3
Composition
4
Flashpoint
5
kg/m3
6
vol. %
°C
1
Solvent with
low flashpoint
C8H12
-18
0.042
1.2
7
Vapour
pressure 20
°C kPa
8
Boiling point
5.8
81
9
Relative density of gas or
vapour to air 2)
10
Ignition
Temperature
2.9
°C
260
°C
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
1) Normally, the value of vapour pressure is given but in the absence of that boiling point can be used (reference 4.4.1d).
2) See 4.4.4.
3) For example, IIBT3
11
Group and
temperature
class 3)
IIT3
Reference drawing;
layout
12
Any other relevant
information and
remarks
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
Hazardous area classification data sheet – Part II: List of sources or release
Sheet 2 of 2
Plant: paint factory (example 10)
1
.
No
2
3
Source of release
Description
Location
Area:
4
5
Grade of
release 1)
Reference 2)
6
Flammable material
Operating
temperature and
pressure
7
State 2)
8
Ventilation
Type 4)
Degree 5)
9
Availability 6)
Zone
type
0-1-2
10
11
Hazardous area
Zone extent m
12
Reference
Any other
relevant
information
and remarks
*Above the
source of
release
**From the
source of
release
*Above
ground level
**From the
source of
release
*Inside vessel
Vertical Horizontal
1
Seal of
solvent
pump
Pump
area
P, S
1
°C
Ambient
kPa
Ambient
L
A
Medium
Fair
1
1.0*
1.5**
Example
No. 2
2
Pool at
floor level
below
solvent
pump
Liquid
surface on
mixing
vessel
Opening of
mixing
vessel
Pump
area
S
1
Ambient
Ambient
L
A
Medium
Fair
2
1.0*
3.0**
Example
No. 2
Mixing
area
C
1
Ambient
Ambient
L
A
Low
Poor
0
*
*
Example
No. 5
Mixing
area
P
1
Ambient
Ambient
L
A
Medium
Fair
1
1.0*
2.0**
Example
No. 5
Mixing
area
S
1
Ambient
Ambient
L
A
Medium
Fair
2
1.0*
2.0**
Example
No. 5
3
4
5
1)
2)
3)
4)
5)
Spillage of
mixing
vessel
C - Continuous; S - Secondary; P - Primary.
Quote the number of list in Part 1.
G - Gas; L - Liquid: LG - Liquefied gas; S - solid.
N - Natural; A - Artificial.
See annex B.
Reference
drawing:
layout
13
*Above
openings
**From
openings
*Above
ground level
**From the
vessel
62
Example No. 11
Tank farm for gasoline and oil:
Not to scale
This example shows one way of using the individual examples Nos. 1, 6, 8 and 9. In this
simplified example, three storage tanks (bunded) (diked) for gasoline (item 3), five liquid pumps
(item 1) placed close to each other, one single pump (item 1), one tank car filling installation (item
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
63
4), two oil tanks (item 5) and one oil/water gravity separator (item 2) are situated within the tank
farm.
The principal factors which influence the types of zones are given in examples Nos. 1, 6, 8 and 9.
Taking into account relevant parameters, (see hazardous area classification data sheets), the
following are typical values which will be obtained for this example:
a = 3 m;
b = 7.5 m;
c = 4.5 m;
d = 1.5 m
The drawing No. 11 is a plan view; for vertical extent of the zones, see examples Nos. 1, 6, 8, and
9.
For details (zoning inside vessels, zoning extent, zoning around tank vents, etc.) see examples
Nos. 1, 6, 8 and 9.
NOTE —It is necessary to use examples Nos. 1, 6, 8 and 9 to obtain the correct zoning of the
interior of tanks and separators (Zone 0) together with zoning at tank vents (Zone 1).
In practice there may be other sources of release; however for simplicity, these have not been
taken into account.
64
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
65
Hazardous area classification data sheet – Part I: Flammable material flat and characteristics
Sheet 1 of 3
Plant: paint factory (example 11)
1
No.
2
Name
3
Composition
4
Flashpoint
5
kg/m3
6
vol. %
7
Vapour
pressure
20 °C kPa
8
Boiling
point °C
9
Relative density of
gas or vapour to air 2)
10
Ignition
Temperature
°C
11
Group and
temperature
class 3)
0.022
0.043
*
0.7
1
>0.7
50
6
*
<210
200
*
>2.5
3.5
>1.2
280
330
>280
IIAT3
IIAT2
IIAT3
°C
1
2
3
Gasoline
Fuel oil
Water
containing
oil and
gasoline
<0
55-65
<0
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
1) Normally, the value of vapour pressure is given but in the absence of that boiling point can be used (reference 4.4.1d).
2) See 4.4.4.
3) For example, IIBT3
Reference
drawing; layout
12
Any other
relevant
information and
remarks
The values are
estimated
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
Hazardous area classification data sheet – Part II: List of sources or release
Sheet 2 of 3
Plant: paint factory (example 11)
1
2
3
Source of release
No. Description
Location
Area:
4
5
6
Flammable material
Grade of Reference 2)
release 1)
Operating
temperature and
pressure
7
8
Ventilation
9
State 2) Type 4) Degree 5) Availability 6)
10
11
Hazardous area
12
Reference drawing:
layout
13
Zone
Zone extent m
Reference Any other relevant
type
information and
0-1-2 Vertical
remarks
Horizontal
1
Seal of
gasoline
pump
Pump
area
D
D
°C
Ambient
kPa
Ambient
L
A
Medium
Fair
1
1.0*
3.0*
Example
No. 1
2
Liquid
surface on
separator
Waste
water
treatment
C
1
Ambient
Ambient
L
N
Low
Poor
0
*
*
Example
No. 2
N
High
Poor
1
1.0*
3.0*
N
High
Poor
2
3.0*
7.5*
Example
No. 8
Example
No. 8
Example
No. 8
*Above ground level
**From separator
*Above ground level
**From separator
*Inside the tank
Tank
C
1
Liquid
surface on areas
gasoline
tanks
Tank
P
1
4 Vent
opening in areas
gasoline
tank
5 Flanges,
Tank
S
1
etc. inside areas
dike bund
of gasoline
tanks
6 Overfilling Tank
SS
1
of gasoline areas
tanks
1) C - Continuous; S - Secondary; P - Primary.
2) Quote the number of list in Part 1.
3) G - Gas; L - Liquid; LG - Liquefied gas; S- solid.
4) N - Natural; A - Artificial
5) See annex B.
3
*Above the source of
release
**From the source of
release
*Above ground level
**From the separator
Ambient
Ambient
L
N
Medium
Poor
0
*
*
Ambient
Ambient
L
N
Medium
Good
1
3.0*
3.0**
Example
No. 8
*3 m around the vent
Ambient
Ambient
L
N
Medium
Fair
2
*
*
Example
No. 8
*Inside dike bund
Ambient
Ambient
L
N
Medium
Good
2
3.0*
3.0**
Example
No. 8
*Above ground level
66
67
Hazardous area classification data sheet – Part II: List of sources or release
Sheet 3 of 3
Plant: paint factory (example 10)
1
2
3
Source of release
No.
Description
Location
Area:
4
5
Grade of
release
6
Flammable material
Reference 2)
1)
Operating
temperature and
pressure
7
8
Ventilation
9
State 2) Type 4) Degree 5) Availability 6)
Zone
type
0-1-2
10
11
Hazardous area
12
Reference drawing:
layout
13
Zone extent m
Vertical
Reference Any other relevant
information and
remarks
Horizontal
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
1
Seal of
gasoline pump
Pump
area
D
D
°C
Ambient
kPa
Ambient
L
A
Medium
Fair
1
1.0*
3.0*
Example
No. 1
2
Liquid surface
on separator
Waste
water
treatment
C
1
Ambient
Ambient
L
N
Low
Poor
0
*
*
Example
No. 2
N
High
Poor
1
1.0*
3.0*
N
High
Poor
2
3.0*
7.5*
Example
No. 8
Example
No. 8
Example
No. 8
*Above ground level
**From separator
*Above ground level
**From separator
*Inside the tank
Example
No. 8
Example
No. 8
*3 m around the vent
3
4
5
6
1)
2)
3)
4)
5)
Tank
C
1
Liquid surface
areas
on gasoline
tanks
Vent opening in Tank
P
1
gasoline tank
areas
Flanges, etc.
Tank
S
1
inside dikebund areas
of gasoline
tanks
Overfilling of
Tank
SS
1
gasoline tanks areas
C - Continuous; S - Secondary; P - Primary.
Quote the number of list in Part 1.
G - Gas; L - Liquid: LG - Liquefied gas; S - solid.
N - Natural; A - Artificial.
See annex B.
Ambient
Ambient
L
N
Medium
Poor
0
*
*
Ambient
Ambient
L
N
Medium
Good
1
3.0*
3.0**
Ambient
Ambient
L
N
Medium
Fair
2
*
*
Ambient
Ambient
L
N
Medium
Good
2
3.0*
3.0**
Example
No. 8
*Above the source of
release
**From the source of
release
*Above ground level
**From the separator
*Inside dike bund
*Above ground level
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
Hazardous area classification data sheet – Part II: List of sources or release (continued)
Sheet 3 of 3
Plant: paint factory (example 10)
1
2
3
Source of release
No.
7
8
9
1)
2)
3)
4)
5)
Description
Opening in
tank roof of
tank car
filling
installation
Spillage at
ground
inside
drainage
channel of
tank car
filling
installation
Oil Tank
Location
Loading
area
Area:
4
5
Grade of
release 1)
Reference 2)
P
1
6
Flammable material
Operating
temperature and
pressure
°C
Ambient
kPa
Ambient
7
State 2)
L
8
Ventilation
Type 4)
A
Degree 5)
Medium
9
Availability 6)
Poor
Zone
type
0-1-2
Zone extent m
12
Reference
Vertical Horizontal
Any other
relevant
information
and remarks
1
1.5*
1.5*
Example
No. 9
*Above ground
level
**From release
2
1.0*
1.5**
Example
No. 9
1.0*
4.5*
Example
No. 9
*Above ground
level
**From release
*Above ground
level
**From drain
channel
*
**
Loading
area
S
1
Ambient
Ambient
L
N
Medium
Poor
2
Tank
areas
*
2
*
*
L
*
*
*
*
C - Continuous; S - Secondary; P - Primary.
Quote the number of list in Part 1.
G - Gas; L - Liquid: LG - Liquefied gas; S - solid.
N - Natural; A - Artificial.
See annex B.
10
11
Hazardous area
Reference
drawing: layout
13
*No hazardous
area due to the
high flashpoint
of oil
68
69
Table C.1 – Hazardous area classification data sheet – Part I: Flammable material flat and characteristics
Sheet 1/1
Plant:
1
No.
Reference
drawing; layout
2
Name
3
Composition
4
Flashpoint
°C
5
kg/m3
6
vol. %
7
Vapour
pressure
20 °C kPa
8
Boiling
point °C
9
Relative density
of gas or vapour
to air2)
10
Ignition
Temperature
°C
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
1) Normally, the value of vapour pressure is given but in the absence of that boiling point can be used (reference 4.4.1d).
2) For example, IIBT3
11
Group and
temperature
class 3)
12
Any other
relevant
information and
remarks
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
Table C.2 – Hazardous area classification data sheet – Part II: List of sources or release
Sheet 1/1
Plant:
1
Area:
2
3
Source of release
No. Description Location
4
5
6
Flammable material
Grade of
release 1)
Reference 2)
Operating
temperature and
pressure
°C
1)
2)
3)
4)
5)
C - Continuous; S - Secondary; P - Primary.
Quote the number of list in Part 1.
G - Gas; L - Liquid: LG - Liquefied gas; S - solid.
N - Natural; A - Artificial.
See annex B.
kPa
7
8
Ventilation
State 2) Type 4) Degree 5) Availability 6)
9
Zone
type
0-1-2
10
11
Hazardous area
Zone extent m
Vertical
Horizontal
12
13
Reference Any other relevant
information and
remarks
70
NOTE—Figure C.1, which appears in IEC 79-10, is not included in ISA-RP12.24.01. Its schematic approach to the classification of hazardous areas can lead to a conclusion that deviates
from the National Electrical Code (NFPA 70) requirement that there be a Zone 1 “transition
zone” between Zone 0 and Zone 2 or unclassified locations, and a Zone 2 “transition zone”
between Zone 1 and unclassified locations.
Figure C.1
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
71
Figure C.2 – Preferred symbols for hazardous area zones
72
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
Annex D (Informative) – Use of combustible gas detection equipment
The text of Annex D has been added. The underlining has been omitted for clarity.
D.1 Provided the conditions of Par. D.2 are met, the installation of combustible gas detection
equipment can be a basis for the following:
a. An inadequately ventilated area containing equipment that could release flammable gas or
vapor can be designated as Class I, Zone 2.
NOTE—If an area contains equipment that may release flammable gases or vapors within the
area during normal operations, gas detectors are not a feasible alternative unless some
degree of ventilation is provided since frequent alarms or equipment shutdowns, or both, are
likely to occur.
b. The interior of a building (or similar area) that does not contain a source of flammable gas or
vapor can be considered unclassified, even though a door or similar pierced portion or all of the
outside of the building is located in a Class I, Zone 2 area, provided the building is of a type
construction that is essentially vaportight; that is, the building will not allow the entry of significant
quantities of outside atmospheric pressure gas or vapor. Buildings made of fiberglass (molded
fiberglass or fiberglass sprayed over wood) or seal welded steel plate normally are used to meet
this criteria, but other construction methods may be equally satisfactory. Penetrations should be
minimized — normally limited to a personnel entry door(s), electrical cable entries, air
conditioning unit(s), and the like. The buildings should contain no windows that can be opened,
and the personnel entry door(s) should be provided with adequate gaskets or weather stripping.
Openings for air conditioning units and windows should be adequately caulked or otherwise
made vaportight. Air conditioning equipment must not introduce outside air into the building.
Entries for cables and other services should be made in a vaportight manner.
D.2 The criteria for use of combustible gas detection equipment to meet the objectives of D.1
follow:
a. the gas detectors are of a stationary type, permanently mounted; portable gas detectors will
not satisfy this requirement;
b. the gas detection equipment is of a type approved or listed by a nationally recognized testing
laboratory (NRTL). It is recommended that equipment meet ANSI/ISA S12.13, Part 1,
Performance Requirements, Combustible Gas Detectors;
c. an adequate number of sensors is installed to ensure the sensing of flammable gas or vapor in
the building (or similar area) in all areas where such gas might accumulate;
d. sensing a gas concentration of 20 percent LFL (or less) should activate a local alarm (audible
or visual, or both, as most appropriate for the location);
e. sensing a gas concentration of 40 percent LFL (maximum) or a gas detector system
malfunction should both activate an alarm (audible or visual, or both, as most appropriate for the
area) and initiate automatic disconnection of power from all electrical devices in the area that are
not suitable for Division 2. The power disconnecting device(s) should be suitable for Class I,
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
73
Division 1, if located inside the building (or similar area); if the disconnecting device(s) is located
outside the building (or similar area), it should be suitable for the area in which it is located.
Redundant or duplicate components (such as sensors) may be installed to avoid disconnecting
electrical power when single component malfunctions are indicated. When automatic shutdown
could introduce additional or increased hazard, this technique of area classification reduction
should not be used;
NOTE—In the case of sensing 40 percent LFL or a gas detection system malfunction,
corrective action to reduce the gas concentration should be initiated immediately.
f. the gas detectors shall be calibrated at a frequency in accordance with the manufacturer’s
recommendations, but at least once every three months. Calibration shall be performed by actual
exposure of the sensor to a known mixture (nominal 50 percent LFL recommended) of diluent
and methane or other gas anticipated in accordance with the manufacturer’s recommendations;
g. user-provided systems bypassing the disconnecting or other “corrective action” devices (but
not audible or visual alarm devices) to allow calibration and maintenance are permitted, provided
the bypass system is utilized only during calibration or maintenance operations, and only while
the area is manned by personnel who are qualified to take corrective action should there be a
malfunction in process, storage, transfer, or similar equipment that potentially might release
flammable gas or vapor into the area. Any systems in the bypass mode must be made
continuously obvious (audibly or visually) to facility personnel; and
h. the building (or similar area) contains no electrically heated parts or components (not
enclosed in explosionproof enclosures) that may operate at a temperature equal to or above 80
percent of the ignition temperature (expressed in degrees C) of the gas or vapor involved unless
the component has been verified by a nationally recognized testing laboratory (NRTL) to operate
below the ignition temperature of the gas or vapor.
NOTE—Electrically heated parts and components could remain at or above the ignition
temperature for some time after de-energization.
D.3 It is recommended that gas detectors be installed, operated, and maintained in accordance
with ANSI/ISA RP12.13, Part II, Installation Operation and Maintenance of Combustible Gas
Detection Instruments.
74
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
Annex E (Informative) – United States major deviations
THE TEXT OF ANNEX E HAS BEEN ADDED.
THE UNDERLINING HAS BEEN OMMITTED FOR CLARITY.
General, 1.1 Scope. ISA RP12.24.01 is a recommended practice as opposed to IEC 79-10,
which is a standard.
General, 1.2 References. The word “normative” was deleted. The references in ISA
RP12.24.01 are both normative and informative. Additional references have been added as
required to satisfy United States deviations.
2.4.4 Grouping and Classification. The definitions of Grouping and Classification were
replaced by definitions from ANSI/NFPA 70, the (United States) National Electrical Code.
2.4.1-2.4.3 The definitions of Class I, Zones 0, 1, and 2 were replaced by definitions from ANSI/
NFPA 70, the (United States) National Electrical Code.
2.20-2.21 The definitions of flammable and combustible liquids from ANSI/NFPA 30, Flammable
and Combustible Liquids Code, have been added.
Additional material from API RP 505 was added, addressing matters not covered in IEC 79-10.
Annex C, Notes. Notes were added to explain that the annex is not entirely applicable to
installations in the United States since ANSI/NFPA 70 requires a Zone 1 “transition” zone
between Zone 0 and Zone 2 locations and a Zone 2 “transition” zone between Zone 1 and nonhazardous locations.
Figure C.1. Figure C.1 was deleted since it is not compatible with ANSI/NFPA 70 classification
methods.
Annex D. Annex D was added as an informative annex to give guidance on the use of
combustible gas detectors to reduce area classification in certain cases.
ISA-RP12.24.01-1998 (IEC 79-10 Mod)
75
Developing and promulgating technically sound consensus standards,
recommended practices, and technical reports is one of ISA's primary
goals. To achieve this goal the Standards and Practices Department
relies on the technical expertise and efforts of volunteer committee
members, chairmen, and reviewers.
ISA is an American National Standards Institute (ANSI) accredited
organization. ISA administers United States Technical Advisory
Groups (USTAGs) and provides secretariat support for International
Electrotechnical Commission (IEC) and International Organization for
Standardization (ISO) committees that develop process measurement
and control standards. To obtain additional information on the
Society's standards program, please write:
ISA
Attn: Standards Department
67 Alexander Drive
P.O. Box 12277
Research Triangle Park, NC 27709
ISBN: 1-55617-649-X